Internal Parasites:

Unfortunately, fish are prone to attacks by a great many different internal parasites, under which we can class as worms and unicellular like Spore Animalcules, Flagellates, Ciliates and ( more remotely ) Fungi and Bacteria. With limited space here I will only address the most common and important parasites, since even the most experienced hobbyist it is easy to get lost in the multi-syllabic names.  Internal Parasites are nasty and very difficult to combat effectively, preventing the spread of these nasty guys is of utmost importance. More often than not due to the low cost of fish and higher cost of medications, euthanizing and replacing the fish is more cost effective, although granted not the first choice for many of us attached to our prized possessions.

This is not ICH, the white spots disease, please do not confuse the two, If you are looking for ICH information and treatment, click here.
The greatest threat to all aquarium fishes is without a doubt the fungus Ichthyphonus/Ichthyosporidium.
Approx. 50%-60% of all fish deaths can be attributed to this nasty critter. This parasite can dwell anywhere, in the eyes, kidneys, liver, reproductive organs, muscles, gills, fins, muscles and even in the brain. Most common symptoms of this parasite is ragged or loss of fins, tumors, lumps or lesions of the skin, pop-eyes and even eyes that fall out of the head, constant gasping at the surface, dark or black body color.
Just about all fish deaths that show no external symptoms can be traced to Ichthyophonus upon autopsy. This critter is especially nasty since it will spread quickly within the tank from open lesions in the skin, infected feces, even passed through diseased gills into your tank water. Soon to be picked up by other members within the community tank, primarily at feeding times. A successful cure for these parasites has not yet been made possible. Best recommendation is to remove suspected fish from the tank and destroy them.

  PLISTOPHORA: (Neon Disease)
Almost exclusively to Neon Tetras (Paracheirodon) is the disease Plistophora. The red color of the stripe tends to change to white, starting at the base of the tail. Then progresses until the stripe disappears all together. There is no know medication for these spores. There is a rapid transfer rate from fish to fish of this disease. Even fry can emerge diseased. The only way to control this disease is to destroy the fish, and sanitize the aquarium and all of its contents completely.  Generally even after sterilization the successful keeping of Neons in a once infected tank is extremely rare. Plistophoria does not affect other species, so using the aquarium is still okay, just with other species.

There is a bacteria among fishes which have a great similarity to human Tuberculosis. However these mycobacterium exist at optimum temperature of 103 deg. F. While the bacteria within fishes the bacteria multiples best at a range of 64-77 deg F. Scientists believe that due to this difference there is no danger of infection from diseased fish to humans. The Tuberculosis germs form tiny lumps or tubercles in the fish.
There are no know chemical cures. Destruction of fish is the only known treatment at this time.

These infections, whose name actually describes the symptoms, are caused by a bacteria of the genus Pseudomonas. Dropsy has been treated in larger fishes by injections of antibiotics, however with the size of aquarium fishes treatment is not feasible. Best treatment is total immersions in antibiotics or destruction and replacement of the fish.

One of the most commonly observed diseases is the protrusion of one or both eyes from their sockets.
A gas forming bacteria causes a bubble to form from within the eye.
Sometimes the wounded/missing eye will heal and the fish can continue to live, but the more humane way of dealing with this disease is to destroy the infected fish. There are treatments  for POP-EYE, see the Medications Chart.

Skin, gill and intestinal worms are found among most fishes and in great variety. Only the gill and skin varieties can be dealt with directly. More often than not a worm can not be detected until it is too late. Some things to look for are reddish spots along the belly indicating an intestinal worm. If you suspect worms they can be eliminated in some species of fish by a brief immersion lasting no longer than 20 minutes in a salt solution (10g aquarium, canning or pickling salt to 1 liter of water, Do NOT use table salt, see the Salt Info Page.).