Freshwater fish are subject to all kinds of diseases. Nasty pathogens that cause disease are in and around our fish in their natural habitat and in your aquarium. These pathogens may be bacterial, viral, fungal, or parasitic. Many are introduced with new fish and many are highly contagious. Fortunately, many of the diseases caused by these agents will manifest themselves with signs that are easy to recognize. However, there are not a lot of treatments available for the home aquarist, and there are no guarantees that your aquarium fish will be saved. Whether or not disease breaks out depends on the resistance of your fish. Poor living conditions weaken your fish, cause chronic stress, and ultimately compromise the fish's immune system.
Anchor worm - A white worm protrudes from a red, agitated area on the fish's body. Infested fish rubs against anything it can, attempting to scratch off the parasite.
Body slime fungus - Protective skin mucus grows white and starts peeling off, as if the fish were shedding or molting.
China disease - Tail fins and other fins begin to fray, beginning at the base of the fin and working its way outward. Infected areas begin to blacken Ventral region begins to turn black.
Constipation, indigestion - Fish is very inactive, usually rests on bottom of the tank. Abdominal swelling and bulging is likely.
Dropsy (kidney bloat) - Abdomen bloats noticeably. Scales stick out like pinecones.
Fin or tail rot - Fins have missing parts and eventualy become shredded. Rays become inflamed and entire fin may be eaten away.
Fish lice - Round, disk-shaped, transparent crustaceans that clamp onto fish and refuse to let go.
Fish pox - Whitish or pinkish waxy film develops over fish's skin and fins.
Fungus - Fuzzy growth, different from velvet because it is more whitish.
Furunculosis - Raised bumps under the scales that eventually rupture and cause bleeding ulcers.
Gill fluke - Gills swell pink and red. Fish spends time at the surface gasping for air. Puslike fluid will be exuded from the gills.
Hole-in-the-head - Fish has white, stringy feces and enlarged pus-filled sensory pores in the head. Also, erosion of the skin and muscles that eventually extends to the bones and skull.
Ich - Raised white spots about the size of a salt granule appear on the body and fins.
Leeches - Long, wormlike parasites attached at both ends to the fish that do not come off easily.
Mouth fungus - White cottony growth on mouth, sometimes spreading to the gills and other parts.
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Pop-eye - Fish's eyes protrude from an inflamed eye socket.
Skin fluke - Localized swelling, excessive mucus and ulcerations on the skin.
Swim bladder disease - Fish swims on its side, upside down, or somersaults through the water. Fish may be found either at the top or the bottom of the tank.
Tumors - Obvious bumps, lumps, and protrusions that sometimes look like blisters or warts.
Ulcers - Large red lesions, boils, dark reddening, and bleeding.
Velvet - Fuzzy yellow or golden areas.
It is very important that beginners use commercially available treatments instead of homemade remedies. Some experts recommend chemicals such as malachite green or potassium permanganate. These chemicals must be handled in very exact dosages. If a fish is overdosed with one of them, it can kill the fish faster than the disease.
Salt bath is the most time-tested cure-all of the freshwater fish world. Sometimes called the progressive saltwater treatment, it is how the hospital tank is most often used. This very simple treatment has been known to cure a number of fish diseases, including ich white spot disease, fungus, velvet, and tail rot. You simply add 1 teaspoon of table salt (not iodized) for each gallon of water to the hospital tank that houses your sick fish. Add the same amount of salt that night and twice the next day, again in the morning and at night. If there is no improvement by the third or fourth day, add 1 more teaspoon of salt each day. On the ninth and tenth days, make progressive waterchanges and check for results. (Adapted from Gregory Skomal. Freshwater Aquarium)