Iodine Deficiency in Discus, by Bryan Kemper, N.A.D.S.

With our modern knowledge of food nutrition we seldom hear much of disease or ill health in humans caused by nutritional deficiencies.   These problems, common several decades ago, have now been all but eliminated by additives in our routine foodstuffs; vitamin and mineral additives are now common place in everything from salt to cereal.

With this background, you can imagine my surprise when I came face to face with a mysterious problem with a group of two year old discus which I had been rearing as breeding stock.   These two dozen fish had been raised on the standard formula of beef-heart, shrimp, wheat germ, spinach and a multi vitamin supplement.   The fish had reached normal size, shown only fair coloration and did not show any sign of sexual activity by 24 months.   At about this time they began to be very timid and hid under plants or behind flower pots and seemed quite nervous.   Gradually a large swelling appeared in the lower part of the gill and after several weeks a grey mottled sac was visible within the gill opening.   The sac grew to such an extent that the gill would not close.

My first reaction was that some large parasite had invaded the fish and was developing an egg sac of massive proportions, which would burst and infect the entire system.   Treatment with the standard medications provided no relief.   A dissection revealed that the sac was connected to the bottom of one of the gill arches, and that it contained amorphous thin walled sacs of a reddish gelatinous substance.   There were clearly no parasite eggs or larvae present and the structure did not appear to be a tumour.   Now I was really confused, I was so convinced of the earlier parasite idea that I didn't look for other solutions.   I wondered if blood parasites might explain this; maybe they irritated the gill filaments and caused the sacs.

Finally, I decided to seek some professional help and phoned a friend at the provincial vet lab associated with Alberta Fish and Wildlife.   She agreed to run a series of tests if I could spare another live fish.   Their tests for parasites (which still was my main focus) were negative, there only thought was that it might be a thyroid problem because of the location of the swelling.

This new line of thought sent me back to the reference books with a different insight and there it was, not under parasites at all, but on iodine deficiency, the fish version of goitre.   The recommended solution was to add iodine to the water and allow it to be absorbed through the gills.   Caution needs to be exercised here as with humans because excesses of iodine are toxic.

I began to add iodine to the water with almost instant results.   The dosage is based upon a stock solution of 0.5 g iodine plus 5 g potassium iodide in 100 ml of water; add 1 ml for each 50 litres of aquarium water.   Make subsequent editions based on water changes.   The fish were more active, their appetite improved and so did their colour, although the latter improved more slowly.   The constant additions of iodine with the water changes proved to be tedious, so I went looking for a recommendation on dietary sources of iodine.

It seems that this deficiency is well known in the sport fish hatchery business, where the recommended addition is 14 mg potassium iodide per kg of food.   Recognizing that this dosage was for flow-through water systems, I decided to reduce the dosage to 7 mg per kg for the discus aquariums.   When I began the food additive I stopped the water treatments with iodine.   Since I began the treatments a total of five pairs have formed, spawned and have been removed to their own breeding aquaria.   All have successfully reared young and the gill swelling has disappeared from all the fish.   I now add potassium iodide to all food preparation (beef-heart base) at the rate of 7 mg per kg of food mix.

References - Untergasser, Dieter. 1989. Handbook of fish diseases. TFH Publications #T5123. 160p.