(This article was originally published in Cichlid News Magazine, Jul-98 pp. 25, It is reproduced here with the permission of author Mary Ellen Sweeney and Aquatic promotions).
Your fish look miserable. Upon closer examination, you find that they are breathing hard, sometimes with one opercle closed, or gasping near the surface. Sometimes they "flash," rubbing against objects because of gill irritation. They may become dark and listless, and are easy prey to bullies. When you examine the gills, they are covered in opaque mucus, and often the ragged gill filaments protrude from under the gill covers. There may be evidence of secondary bacterial or fungal infections. Perhaps you have already lost a few fish suddenly after a short period of rapid breathing. These are all symptoms of gill flukes (Dactylogyrus spp.).
Gill flukes are monogenetic trematodes that need no intermediate host to reproduce. They are common aquarium pests, especially, it seems, in the cichlid aquarium. Egg-laying adults are 2 mm in length with damaging anchors and hooks. Their toughshelled eggs hatch in 1-5 days, often requiring repeated treatments to control serious infestations.
Adult fish normally tend to have a few flukes, and this minor population is considered like a few fleas on a dog mildly annoying, but not an immediate emergency. Fry and juvenile fish, however, are often devastated by a fluke infestation, and often whole broods are lost just when they have started to get a little growth on them. It is not unusual for a whole tankful of dime (or nickel-sized) discus fry, for example, to suddenly start hanging at the surface and die off a handful at a time over a few days time.
In a stressed aquarium, whether from lack of maintenance or an in creased fishload, the fluke population can reach epidemic proportions. The flukes feed on mucus, skin, and occasionally blood, and they just thrive in deteriorating water conditions. To treat flukes, first and foremost, give the aquarium a good maintenance session with a 50% water change and attention to the substrate, filter media, and inside glass. This to me is the most important part of the therapy, and cannot be avoided. You will already have, through this act, reduced the fluke population enormously, in addition to reducing bacteria and other toxins in the water, increasing the oxygen, and preparing the water for further treatment.
There are several ways to attack flukes depending on the circumstances. Especially where you wish to treat the entire tank, a simple salt treatment is often the safest and most prudent choice. The dose is four ounces of salt per ten gallons of water, maintained over three weeks to prevent reinfestation with newly-hatched flukes. Salt treatment is not recommended for tanks containing Corydoras and other salt sensitive fishes.
Formalin and commercial formalin/malachite green combinations are also used to combat gill flukes. These treatments are a bit more challenging and require careful attention to risk and safety factors. To use formalin and compounds containing formalin, again, begin with a scrupulously clean tank with added aeration. Formalin is added to the aquarium at three drops per gallon in soft water and up to six drops per gallon in hard water for an 8-hour period followed by a 50% water change. Alternatively, a bath of up to 15 drops of formalin per gallon can be applied for 30 minutes but only with vigorous aeration and constant supervision. Formalin/malachite green combinations are used according to manufacturers' directions; of the three approaches, this is probably the most effective. Due to synergistic effects, formalin and malachite green together are more powerful than either is alone. Up to three treatments at weekly intervals may be required to knock down persistent fluke problems.
To prevent future fluke infestations, quarantine every fish before introduction to the aquarium and keep up with scheduled water changes and maintenance. If gill flukes are a constant concern, especially in large tanks that are difficult to treat, consider an ultraviolet sterilizer to eliminate not only gill flukes, but other external parasites and algae as well.
And one final thought . . . For those fry that keep dying off, a short bath of 10 minutes in three drops formalin per gallon of water with an airstone, followed by transfer to a fluke free rearing tank may make all the difference in their survival rate.