This is an excellent article originally published in June 1991 "OUR DISCUS" North American Discus Society.
"Water Quality Management in The Discus Aquarium" by Web Wheeler
1.0 Introduction - When discus were first introduced to the aquarium hobby, aquarists would sometimes observe small holes in the heads of their fish which would disappear after a water change was made. From this observation, the conclusion was drawn that this condition was caused by poor water quality. Today, it's generally felt to be caused by a vitamin and mineral deficiency specifically vitamin C and D and the mineral calcium. This belief, however, does not change the original observation that water quality has an important effect on discus health.
The purpose of this paper will be to discuss the importance of water quality in the discus aquarium and to discuss the ways in which the discus keeper can alter the water quality to his or her advantage.
2.0 Why be Concerned with Water Quality?
2.1 The Natural Condition - In my research on water quality management, I was struck by the adverse conditions that discus have adjusted to in the wild. Theses are: high temperatures combined with low pH and mineral content of the water. I would like to expand on these natural conditions with the hope that this will help us better understand why we must be more concerned about water quality in the discus aquarium than we are about water quality in general. To do this, I would like to illustrate some of the problems that a discus must cope with from a physiological point of view.
First, water is fifty times more viscous than air and eight hundred times denser. Water holds far less oxygen than air at saturation (5 to 10 ml of oxygen per liter of water vs. 210 ml per liter of air) with warm water holding less oxygen than cool water. Thus to extract the same volume of oxygen, the ventilation rate of a fish at 20 degrees centigrade is 28 times that of an air breather. The problem is made worse by the fact that the concentration gradient of carbon dioxide is much less for a fish than it is for air breathing animals - this is especially the case with discus. Because discus water is usually low in carbonates there is less buffering of carbon dioxide than that found in harder waters. This means that a discus has to work much harder than you or I to breathe in oxygen and to exhale carbon dioxide, and somewhat harder than other cooler water fishes. Thus aeration is of greater importance in the discus aquarium, and because digestion consumes large amounts of oxygen, it would be a good idea to feed smaller meals at more frequent intervals than one would with other fish.
Second, because discus waters are low in mineral content, a discus will have greater difficulty in maintaining its body chemistry than would fish living in harder waters. The tendency will be greater for a discus to absorb water and to loose salt to its environment. Although it has been reported that discus do not like sodium chloride added to their water, and I have observed that they do not care for it in their food, perhaps trace amounts of other salts such as iron citrate and calcium sulfate may be beneficial.
2.2 Incidence of Disease - People and fish are very different morphologically, yet we share fairly similar physiology: we both eat, breathe, rest, and suffer from stress. When we are stressed we become more susceptible to illness. If the environment we live in is stressful, we are more likely to get sick. Discus, like people, can tolerate environmental stress for short periods of time and show no ill effects, however, environmental stress over a long period will take its toll.
Where environmental stress is concerned, water quality is of primary importance. By maintaining optimal water quality, our discus will not be stressed by their water conditions, and will be able to recover from injuries, parasitic infections and diseases which could otherwise kill a fish already stressed from poor water conditions.
2.3 Growth Inhibiting Factors - Experimental studies have proven that the size of the aquarium, or water volume, is not related to fish growth. Rather, it is mostly nutrition and water quality that determine size and growth rate.
2.4 Fertility and Reproduction - The effect of water quality on discus fertility and reproduction is not well understood, but several theories have been put forth. Some discus keepers feel that water having a high calcium carbonate content inhibits discus fertility. This could be because the sperm of the male is killed by calcium carbonates, or because of possible embryo suffocation from carbonate precipitation on the surface of the egg membrane. In addition, some discus keepers feel the tendency for parental discus to eat or raise their young is related to water quality.
2.5 Esthetics and Ethics - Most of our enjoyment of discus comes seeing this magnificent fish in its prime. Discus will only look and act their best under optimal water conditions.
In addition, we discus keepers are responsible for the life and health of a living animal. To carry out our responsibility, we should provide our discus with the best possible care that only comes from attention given to the conditions they are kept in. All to often I hear stories about how tough discus are - about how they survive even though they are extremely neglected. Such stories do not impress me.
3.0 What is Good Water?
3.1 Ions and Ionic Balance - There are two factors which one should consider in the makeup of water for the discus aquarium: total salt content (or conductivity) and ionic balance. Nowadays, various ion exchange resins and expensive conductivity meters, used for the purpose of lowering and measuring total dissolved salts, are fashionable. However, unless one is able to determine the ratio of bivalent to univalent ions as measured by the concentration of carbonates and sulfates vs. remaining salts calculated as mg/litre of sodium chloride one is doing little more than playing russian roulette.
When salts such as calcium sulfate or sodium chloride ionize, or dissolve, they do so as bivalent (expressed as degree hardness) or univalent ions. It is important that aquarium water contain roughly equal proportions of bivalent (calcium, magnesium, sulfate) to univalent (sodium, potassium, chloride, nitrate) ions. Therefore, a chemical analysis (usually available from your local municipal water works department) should be obtained before one attempts any alteration of one's tap water with the use of ion exchange resins. From this analysis you will be able to determine what effect carrying out a partial ion exchange will have and can make adjustments accordingly. This of course also applies if it is your intention to completely demineralize your water, only to partially reverse this process by the addition of entreated tap water.
3.2 Organic Molecules - There are many organic compounds that can be found in aquarium water. These range from harmful pesticides, perfumes, organic solvents, ammonia, phenols, urea and other waste products of metabolism to beneficial hormones and organic acids found in peat.
3.3 Dissolved Gases - Most all tap waters contain dissolved gases. Proper aeration before use will generally assure us that harmful gases such as chlorine, ammonia and nitrogen are driven out and replaced by oxygen and small amounts of carbon dioxide. While on the subject of dissolved gases, a word of caution should be noted when adjusting pH. One should always aerate water in which a high pH has been lowered with strong acids before addition to an aquarium containing fish. This is because lowering the pH will liberate large amounts of carbon dioxide from waters containing a high carbonate content which could be dangerous if not fatal to your fish. For the same reason, any pH change occurring in the aquarium should be carried out with care.
3.4 Particulates - While particulates may occur in the water, as long as they are not of a toxic or decomposing nature, they should do little harm and can be removed with the use of mechanical filtration devices.
4.0 Water Treatment
4.1 Reverse Osmosis - Reverse osmosis (or R. O.) units have recently gained much popularity because of their ease of use in removing dissolved minerals from tap water. There are various models which tolerate a range of input water conditions and produce various ratios of product water to reject water. All units have similar things in common: they operate on membranes which will eventually clog and require replacement, they all need pressure to operate, and none are ionic specific. In general, reverse osmosis units are best purchased for the treatment of tap water prior to its use in the discus aquarium.
4.2 Ion Exchange - Ion exchange columns have a number of advantages over reverse osmosis units, however, they are much more complicated to design and use. The advantages of an ion exchange column over an R. O. unit are: the ionic exchange resins can be regenerated, they are ion specific (can be designed to remove only certain ions such as ammonium or nitrate), and they can be used inline with other water filtration devices.
In general, there are three classes of ion exchange resins: neutral, acid and base. The acid and base resins are further classified according to their strength: high, medium and weak.
Neutral resins replace, rather than remove, one or more ions from solution. For instance, a neutral resin could be used to replace only nitrate with chloride ions, or another neutral resin could be used to replace all negative ions with chloride ions (such as in water softening equipment). In most cases neutral resins should not be used in the discus aquarium. The one exception is when one wishes to soften the input water to an R. O. unit in order to prolong the life of the R. O. membrane.
Acid resins are used to replace positive ions (calcium, magnesium, sodium, potassium) with hydrogen resulting in the production of acidic solutions, while basic resins replace negative ions (chloride, carbonate, nitrate, phosphate, sulfate) with hydrogen resulting in the production of alkaline solutions. If one uses a mixture of acidic and basic resins, completely demineralized water may be produced. On the other hand, if one started with hard water consisting mostly of carbonates, a partial ion exchange could be carried out with an acidic resin to produce softer, but not completely demineralized water.
In older discus aquariums, where the water is rather acidic and loaded with ammonium ions, I have placed the mildly acidic resin Amberlite IRC-50 in an outside power filter to remove ammonium ions before they could become toxic due to an increase in pH. This resin also kept my water completely free of calcium carbonates.
4.3 Molecular Absorption - The process of molecular absorption is used mostly to remove organic compounds from the water. For this purpose, ion exchange resins can be used as in the products "Chemipure" and "Polyfilter". One could also use activated carbon, however, there is great variation from one carbon to another, and there is no way to determine the suitability of a particular carbon for use in the aquarium without actual experimental results.
4.4 Mechanical Filtration - For functionality and ease of use, mechanical filters offer numerous advantages. An easy to clean outside power filter using a wad of polyester fiber is the best choice for this purpose. One should not use diatomaceous earth which contains calcium carbonate and may irritate the discus.
4.5 Nitrification - When discussing the subject of aquarium water quality, the first thing that comes to mind is ammonia. We are all aware that ammonia, produced as a result of fish metabolism and from the decay of organic matter, is toxic to fish. We use biological filtration to break down ammonia into less toxic nitrites and nitrates, but in the discus aquarium this may not always occur.
At pH values below 6.5, which often occur in soft poorly buffered discus water, nitrification stops. This generally will not cause a problem because at such low pH values the non toxic ammonium ion predominates. However, as pH rises, the non toxic ammonium ion is converted to highly toxic ammonia and fish mortality may occur. This is most likely to happen when making a large water change in tanks having a low pH.
4.6 Denitrification - Denitrification is the process whereby nitrates and nitrites are converted by anaerobic bacteria to nitrogen gas, which is in turn removed by aeration of the water. This process has gained some favor with recent advances in marine reef keeping, however, before attempting this technique, the discus keeper should be very familiar with the entire process.
Denitrification must be carried out in the absence of oxygen and with suitable food source for the anaerobic bacteria. Frequent monitoring of denitifier effluent water is also necessary to insure that the process goes to completion. It is possible, for instance, for the process to stop early with a resulting increase of toxic nitrite. Another problem is with the production of hydrogen sulfide in waters having a high sulfate content.
4.7 Additives - If I could choose only one product to add to my discus water, peat moss would be it. The use of a good quality peat can lower your pH. reduce hardness, detoxify the water, add beneficial hormones and fungicides to induce spawning and aid in the prevention of egg fungus, add humus and tannins which darken the water and reduce algae growth, buffer the water, and increase redox. There are many brands of peat available and the one you select should not contain pesticides or fertilizers. Furthermore, it should be crumbly and should not contain volatile organic acids (acids which break down when the water is aerated).
4.8 Gas Exchange - Recently, two new products have made their way into aquarium use. They are the bio-filter and the oxygen reactor. The bio-filter has been the subject of much attention, and I believe that the oxygen reactor will prove to be very beneficial as well, although I do not currently have any practical experience with this device.
While the bio-filter has enormous use as a facilitator of nitrification in many aquariums, because of the low pH in discus tanks, I believe that its greatest asset is in the area of beneficial gas exchange, i.e. the addition of oxygen and removal of carbon dioxide. For this purpose, the oxygen reactor may prove to be an even better product.
5.0 References
"Diseases of Fishes Book 5: Environmental Stress and Fish Diseases" by Dr. Carl A. Wodomoyar & Dr. Fred P. Meyer & Dr. Lynnwood Smith, TFH Publications.
"Seawater Aquariums the Captive Environment" by Stephen Spot, John Wiley & Sons (Publisher)
"Water Chemistry for Advanced Aquarists" by Guido Huckstedt, TFH Publications.
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Understanding The Modern Bio-Filter by Web Wheeler, Nautilus Aquatics, Inc. originally published in "Our Discus" Vol 4 Issue 1 1990, N.A.D.S.
Introduction - Being a rather late night person, I often find myself sitting in front of the TV after all the aquarium lights go off. At that time of night you can watch almost any kind of program you wish, as long as it's a talk show. Now most of you are probably wondering what has all of this got to do with biological filtration? Nothing! It just seemed like a good way to start this article and to illustrate a point that I am going to make. Whenever a guest appears on one of these talk shows, they first present some credentials, then they give a plug for their commercial endeavors, and then they tell you an interesting story. I will now attempt to do all of the above.
First, I have been a hobbyist for some twenty odd years. So what? you say, that just makes me an old hobbyist! Well, secondly, I am an active participant in several fish clubs, in which I would also encourage you to participate. Thirdly, I have a fish business. I don't sell fish, but I do try to provide quality filtration, lighting, and water chemistry products. That's the commercialism which leads me to my final credential: being in "the business", I have made it my business to learn as much as I can about filtration products that are available.
Importance of Good Filtration - I am going to assume that the importance of ridding the aquarium of toxic waste products as quickly as possible is well understood by everyone, and that a long discussion about the importance of good filtration is unnecessary. I would like to emphasize a few of the results of poor quality filtration however. First, a high level of ammonia in the aquarium can kill fish outright. This is especially noticeable with baby fish which will quickly die if left in foul water, or with larger fish being shipped in small bags over a long period of time. Secondly, even a low level of ammonia in aquarium water can cause environmental stress which will reduce your fishes resistance to disease and parasites. Please refer to the book "Diseases of Fishes", book 5: Environmental Stress and Fish Diseases, TFH Publications, for more on this subject.
Just so that we don't leave the subject of good filtration on a negative note, I would like to mention the many testimonials which can be found throughout aquarium literature citing good filtration and water quality as the secret to a successful aquarium.
Conventional Filtration Contradictions - Before beginning a discussion of the applications of filtration techniques to the aquarium, I would like to present two contradictions that often negate the intended results, I.e. improved water quality. The first is elementary, but worth noting since it occurs often. Mechanical filters which are left uncleaned do nothing more than move debris from one place to another, and are a waste of electricity. Many hobbyists would do well to keep this in mind when they remark on their filter's performance: "Boy, that filter really works...look how dirty it is!" Please don't make the mistaken assumption that I don't believe in mechanical filtration. My feelings are quite the contrary, but that in order for a mechanical filter to work, it must be cleaned regularly.
The second contradiction is more complicated but you don't have to be a rocket scientist to appreciate it. For many years, biological filtration has been carried out in the aquarium through the use of sponge and undergravel filters. They work quite well except when they're really required to work! Let me illustrate this point with the following example: one of your aquariums springs a leak and you are forced to remove the existing fish and combine them with fish in another aquarium. As you would expect, with more fish in your remaining aquarium, there is an increase in ammonia being produced. But, on the other hand, you also expect the bacteria in your sponge or undergravel filter to multiply as a result of the more plentiful food supply, ammonia, and take care of the problem. This is exactly what happens, but with the following results. The bacteria multiply and begin to break down the additional ammonia according to the following equation: 4NH3(ammonia) + 7 O2(oxygen) - 4NO2(nitrite) + 6 H2O(water). It is not necessary to understand the bio-chemistry behind this, but rather note that oxygen is needed in the process. Even under the best circumstances; I.e., that your remaining aquarium is not overcrowded, you have increased the competition for oxygen in the equation bacteria vs. fish. The contradiction here is that the better the sponge or undergravel filter works; I.e. the more bacteria it produces, the greater the competition for oxygen becomes in bacteria vs. fish. Since bacteria are capable of living under extremely low concentrations of oxygen, there's no contest - the fish always lose. In the above example, the worst case is that your remaining aquarium will be overcrowded, and increasing your conventional filtration will only serve to exchange a fatal concentration of ammonia for an equally fatal oxygen depletion of the water!
Background of Trickle Filtration - You may argue, quite correctly, that the main problem with traditional biological filtration is that both bacteria and fish are in competition for available oxygen. Therefore, one objective in the design of any biological filter must be to remove the competition for oxygen between bacteria and fish. If this can be done successfully, then both can co-exist and thrive without interfering with the well being of each other. Of course there are many other design criteria for a good bio-filter, a few being ease of maintenance, size vs. performance efficiency, reliability, functionality, cost, etc. I will discuss some specifics later in this article, but for now I would like to give you a bit of the historical development of the modern bio filter.
Contrary to popular belief, George Smit did not invent the trickle filter, nor his countrymen, nor did the Germans. In fact, the trickle filter was originally developed by the Japanese who needed large filtration devices for their Koi Pond Gardens. Their original design consisted of slotted wooden towers, some standing ten to twelve feet in height. Water was introduced at the top where it would trickle over the wooden slats until it reached the bottom, draining back into the pond. This process greatly enriched the water with oxygen and at the same time caused large numbers of bacteria to colonize over the wooden slats. From this beginning technology, it is not clear whether the Germans or the Dutch were the first to adopt trickle filtration to aquarium use. However, it is known that trickle filtration has been widely used in both Germany and Holland for the past two decades.
Advantages of the Modern Bio-Filter - The modern bio-filter has two significant advantages over conventional sponge and undergravel filters: The first advantage is that competition for oxygen between bacteria and fish is eliminated. This is accomplished by placing the biologically active portion of the filter at the air/water interface. I will explain this concept. As you are probably aware, gas exchanges in an aquatic environment take place at the air/water interface. In a conventional aquarium, this is largely at the surface, however, a small amount of gas exchange may also occur within air bubbles as they rise from an airstone or filter lift tube. With the addition of a bio-filter, the air/water interface increases proportionately to the surface area of the filter media. Simply put, if one had an aquarium with a bio-filter holding 10 gallons of filter media and each gallon of media had 10 square feet of surface area then the new air/water interface would equal the surface area of the aquarium plus 100 square feet of virtual surface area in the bio-filter. This increase is significant when compared to a conventional 100 gallon aquarium having roughly 12 square feet of surface area. By placing a biological filter at the air/water interface, oxygen needed by nitrifying bacteria is much more readily available, at little or no expense to the other aquarium inhabitants, since it can be taken more directly from the surrounding air than from the surrounding water, which is not the case with conventional biological filtration.
The second advantage of the bio-filter is another result of the additional virtual surface area provided to the aquarium. As the surface area of an aquarium increases, so does the level of gas exchange. I have thus far been primarily concerned with oxygen consumption in the aquarium, but this is only half of the respiratory process. The other half, excretion of carbon dioxide, is equally important. One of the laws of gas physics is that a gas has a natural tendency to diffuse from an area of greater concentration to one of lessor concentration, and to reverse that tendency takes energy. The significance of this priciple in an aquarium environment is that as the concentration of carbon dioxide increases in the aquarium water, the more difficult it is for the inhabitants to respire their CO2. In a situation where there is a higher concentration of CO2 in the aquarium water than in the blood stream of the animal, the natural tendency would be for CO2 to diffuse into the animal rather than out of it! The only way for an animal to overcome this tendency is to consume more energy for respiration. If the energy level for respiration is greater than what the animal is able to sustain, it will asphyxiate regardless of the level of oxygen available. To summarize the above, the second advantage of the bio-filter is that it provides additional virtual surface area to the aquarium for beneficial gas exchange, greatly reducing CO2 stress, and under extreme circumstances, CO2 asphyxiation.
Selection of a Biological Filter Media - There is currently a great deal of debate as to which filter media is the best to use in the modern bio-filter. I would like to define some terminology, give some guidelines, and then let you be the judge of which filter media is the best.
The significant parameters which can be used to determine the performance of the filter media in question are:
(1) Surface Area, (2) Void Space, (3) Wetability, (4) Tendency to Channel and (5) Tendency to foul.
Surface area per unit volume, usually given as square feet per gallon or square feet per cubic foot, is one of the most often quoted fiquers as being the media's main selling point. This should not be considered unless other relevant information is also available. The magnitude of this figure will give some indication of the available surface area for benficial gas exchange, and for bacterial growth. German literature often states that the quantity of filter media for a system should be approximately ten percent of the overall system volume; I.e., 1 gallon of media for every 10 gallons of water; however, this figuer is derived from the use of only one manufacturer's media, whose surface area per gallon is much different than that of other available media. What we really need to know is how many square feet of (media) surface area per gallon of water is necessary. In general, I have found that 1 square foot (media) surface area per gallon of fresh water, and 1.5 square feet of (media) surface area per gallon of salt water is quite adequate. These fiquers may even be a little on the high side depending upon the other characteristics of the media which will be discussed shortly. A sample media quantity determination using the above fiquers would go as follows: I have 100 gallon fresh water aquarium, therefore, I will require media having a total of 100 square feet of surface area. I find a media having 10 square feet of surface area per gallon, concluding that I will need 10 gallons of this media and a bio-filter large enough to contain it.
Void space is another important parameter which should also be considered when selecting bio-filter media. Essentially, "void space" is an expression of the air space remaining after a container has been filled with media. An expression such as void space = 90% means that a container filled with media is still 90% air. In general, the less void space a media has, the greater the surface area of the media will be. Take for example sand and gravel, which has a low void space, and a high surface area. This is not very desirable media for bio-filters, however because the smaller the media's void space, the less its capacity for beneficial gas exchange will be. A good void space figure is around 85% to 95%.
Wetability is a figure that is almost never provided by the manufacturer and can only be guessed from observation. The wetability of a media is simply its ability to get wet all over. In general, media having a ball or barrel shape wets more easily than media having a large flat surface. This is because water flowing over round objects tends to to wet them more evenly, while water flowing over flat objects tends to only wet the top half, leaving the bottom half relatively dry. An example of a filter media having good surface area, good void space, but poor wetability, is plastic drinking straws. It can be observed that, even though drinking straws have a high surface area and good void space, unless the straw is oriented just so, water seldom gets a chance to flow through the inside of the straw, and if it does, it is almost certain not to touch the top half of the inside.
Another factor to consider when choosing bio-filter media is the tendency of the mediato disperse or channel the flow of water. A good media should not disperse or channel too much or too little. Some filtration media have a tendency to disperse the flow of water outward, and then down, the sides of its container, with a resulting relatively dry cone shaped section of media in the middle. At the other extreme is a media which channels the flow of water straight down in streams resulting in poor wetting of the surrounding media. Again, dispersion vs. channelization are almost never provided by the manufacture and can only be guessed at.
The last factor to consider in choosing a bio-filter media is its tendency to plug and foul, which should be avoided. Unlike a mechanical filter which is best cleaned on a regular basis, once a bio-filter is established it should not be disturbed. Any media that tend to plug and foul would have to be periodically removed and cleaned, or disposed of, resulting in a potential major biological upset in the aquarium.
Bio-Filter Design Criteria - Having discussed the performance objectives of the modern bio-filter, elimination of oxygen competition in bacteria vs. fish and optimal gas exchange, we are now in a good position to consider design criteria which will give us these objectives.
As I'm sure you've guessed, the selection of a good filtration media plays a major part in the overall performance of a bio-filter. A properly designed bio-filter will enhance that performance, while a poorly designed bio-filter will often diminish it. As far as media performance is concerned, there are three factors which should be considered in the bio-filter design.
(1) There should be more provision for mechanical pre filtration of the water prior to entry into the biological chamber containing the filter media.
(2) The bio-filter should disperse the incoming flow of water evenly over the top of the filter media.
(3) The bio-filter should facilitate good ventilation of the media so that there is plenty of fresh air surrounding it at all times.
I would now like to discuss the above three guidlines in greater detail.
Mechanical Pre-filtration - In early system designs, a hole was drilled near one of the back corners of the aquarium. A bulk head fitting was placed into the hole along with a perforated standpipe wrapped in filter floss or DLS (a spirally wound floss material). The whole corner section was then partitioned off so that water had to flow over the partition and through the floss material before leaving the aquarium via the standpipe and connecting hose below. This process is illustrated in many texts on "mini-reef" aquariums, for anyone who might be unfamiliar with it. Lately, overflow boxes working on various leveling syphon principles have been developed which eliminate the need to partition and drill the corner of the aquarium. Some of these overflow boxes also have provisions for some sort of mechanical pre-filtration as well. An alternative to placing the mechanical filter in a corner of the aquarium, or in an overflow box, is to place it on top of the bio-filter itself, just above the filter media. The arrangement, if properly designed, affords a larger pre-filter, easier maintenance, and less potential for disaster should the pre-filter suddenly become clogged.
Water Dispersion - There are many ways one could disperse water evenly over the surface of the biological media; however, because of various filter size constraints and costs involved, only two have gained much popularity. They are the spray bar and the drip tray. The design and action of a spray bar is very similar to that of the washer bars found in dishwashers. The unit consists of a perforated rotating arm connected to a Teflon washer or ball bearing joint. Water entering the unit causes the perforated arm to rotate over the top of the media, thus dispersing the water flow. There are several things to look for when selecting a spray bar. First, the quality of the joint can vary from one bar to the other, and, in the early days, there were many problems with joints seizing up over time. Secondly, unless the bar can be adjusted for flow rate, increasing the flow of water will also increase the speed at which the bar turns. A rapidly spinning bar causes water leaving it to fly outward, missing the center section of the biological media below. The alternative to the spray bar is the drip tray, which is nothing more than a shallow tray with an evenly perforated bottom. In operation, water enters the tray and then flows across the bottom. As water fills the tray, it also begins to drain down through the perforations over the media. The tray should be designed so that there is always a slight head of water inside the unit, insuring even drainage. Filters employing the drip tray method of water dispersion may also employ a mechanical pre-filter, then there is little danger of the drainage holes becoming blocked by debris.
Ventilation - All things considered, proper ventilation of the filter media is the most important function of the bio-filter, yet usually the least emphasized. Some manufacturers provide one or more nozzles at the sides of their units where air can be injected from an external air supply. This will work well provided that the nozzles are placed properly and connected to a good air source. An alternative to forced air ventilation is ventilation by convection, where a portal is provided at the top of the bio-chamber and another at the bottom. In this arrangement, the bio-chamber behaves like a chimney, causing ambient air to flow through the unit.
Besides providing a suitable container for biological media, the modern bio-filter can also serve other usefull functions as well. Some of these are: providing a place for chemical filtration to occur, for example organic matter absorption through the use of activated carbon, or water softening with ion exchange resins. The biological filter can also provide a place for additional accessories such as water heaters, thermometers, pH controllers, etc. all of which can be very unsightly in a beautiful display aquarium. Lastly, some bio-filters include float valves or switches that can be connected to an external water supply, facilitating automatic replacement of water lost through evaporation, and automatic water changes when used in connection with drainage facilities.
Conclusion - To minimize the impression that I am endorsing any particular manufacturer's products, including my own, I have intentionally left out pictures and diagrams of filtration units, but do encourage those of you contemplating a purchase, or the construction of your own unit, to study the many bio-filter advertisements available in light of the above information.
