Aquarium parasites can be reduced, in many cases, by careful use of a hospital aquarium where all new fish are quarantined before being added to the actual community.
If the fish show a problem in the hospital, then it can be prevented from jumping and infecting the rest. It is possible for some parasites to be actually in the environment in low levels that are impossible to detect.
Possibly the most common, Ichthyopthirius (ICH, white spot disease) is often in the aquarium and can only become evident when stress is applied.
This accounts for many instances where no new fish or other conditions have changed, but a temperature fluctuation allows the free swimming phase to get anchored on the fish and explode.
More often, though, these parasites are imported with new fish. They can cause a serious problem in an established aquarium.
The common symptoms are a few or many raised white spots, the size of grains of salt. Fish may scratch against gravel or stationary objects. The only thing I use or recommend for treating fish ich disease is Nox Ich because it simply works.
Oodinium (Velvet) is another serious problem for fish. Of all common fish ailments, velvet is the most difficult to diagnose and is often confused with ICH. The velvet cysts are considerably smaller and yellower.
The single celled parasite that causes the problem is Piscinoodinium pillulare. Upon close examination colonies thrive on the fish and appear as gently moving patches.
Velvet is fatal to young fish and must be treated immediately. Adults often carry it without showing distress. Affected individuals, when used for breeding may cause problems. The fry will perish from the disease.
Fish lose normal color, scratch against gravel or stationary objects and appear to be sprinkled with golden talcum powder.
As a treatment for Velvet I would buy the Nox-Ich because the active ingredient is Malachite Green and it kills the Velvet - at least occasionally - often it sticks around if the parasite is not in the MG bath for at least 7 days. Make sure you are aggressive going after this one.
One problem often seen with Cichlids is Hole in the Head Disease, this is normally caused by a parasite inflaming the openings of the lateral line.
I don't have a lot of personal experience with Hole In The Head Disease but I know it can be tough. I would probably try a shotgun approach with many treatments on a quarantined animal. My choices would more than likely be Clout, Furan, and Maracyn.
There are other parasites that are larger. Fish lice (Argulis) and Hydra can be imported into the tank with infected fish or sometimes with live plants.
They can be quite a pest, but are large enough to be seen in most cases. The chemicals used to treat them effectively are banned in some countries, but even worse they are dangerous to many fish as well.
The best remedy for lice and hydra is to get tweezers and pick them off.
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Beginning Aquarium Tips