fish sickness and nursing
Fish diseases are difficult to diagnose and treat. Their smaller bodies, immersion in an liquid environment, and inability to communicate makes this true. Prevention is much better than cure. That said, fishkeepers can and do routinely diagnose, medicate and cure many fish diseases.
Avoid disease by keeping fish unstressed
Fish often fall sick when their immune systems weaken because of stress - which can come from lack of hiding places, overcrowding, poor water quality, excessive handling, or being forced to live with more aggressive or hyperactive fishes. The diseases are often from germs which are present in virtually every aquarium, but which are easily fended-off by a healthy, unstressed fish.
Avoid disease by being careful with new fish
Once you have an established tank, you should take steps to avoid having new fishes introduce disease to it. The first step is to look at the tank in the shop carefully, before you buy the fish. Do the fish in it look healthy? Are there any dead fish in the tank? Any fish that are having trouble swimming, lying or floating listlessly, or any that are continually bumping into walls and other fish? Do any have stringy white or translucent poo hanging from them, or worms hanging from them, or unusual white fuzz or spots on their bodies? All these indicate disease. Do you notice the store manager dosing the tank with medicine? Avoid any fish from such tanks.
Once you do buy the new fish, put them in a smaller (6-10 gallon) quarantine tank and leave them there for 2 weeks. If they still seem healthy, you can move them to your main tank. Make sure your quarantine tank is cycled, or close to cycled. You ensure this by moving some of the gravel from the main tank to the quarantine tank, and perhaps some extra biological media that you keep in the main tank to it as well. Don't move too much, or you could cause a mini-cycle to happen in the main tank by depleting too much of the beneficial bacteria.
Catch disease early
Any significant change in fish appearance or behavior should alert you to the presence of a disease. Listlessness in a normally active fish, a fish that was a normal part of a group being picked-on and chased away by its groupmates (a natural quarantining instinct), disinterestedness in food, spots or fuzz on the body, colorless or white or color-banded poop hanging from the fish, abnormal bloating of the body, fin rot, wounds or ulcers on the body, abnormal posture (vertical, for instance) or movement (whirling or erratic motions), and unusual levels of lurking near the surface are all symptoms of disorders.
Immediately quarantine fish you suspect to be sick
Fish diseases spread from one to the other rather quickly, and mortality rates can be pretty high. If you see any signs of a sick fish, move it as soon as possible to the quarantine tank. Speed is of the essence. The quarantine tank is also smaller, so it is cheaper to medicate than your larger main tank. Never take gravel or nets you use in a quarantine tank to use in the main tank. If fish deaths happen, throw the gravel away, or boil it before reusing.
Medicinal options of first choice
For many diseases, but not all, raising the water temperature (to 82F) and adding slight amounts of aquarium salt (usually around $2, normal table salt is unsafe to use with fish) can be helpful.
If disease has progressed though, stronger remedies are required. Many swear by two medicines made from natural ingredients - Melafix and Pimafix. Each is about $6, and can be useful for a variety of problems. Melafix is used for bacterial, and Pimafix for fungal infections. Neither affects the beneficial bacteria in your tank. They will make the water frothy and have a strong peppermint oil like smell. You have to redose the water every 24 hours. Melafix and Pimafix can safely be used together.
Stronger still are the traditional antifungal and antibiotic medicines - Maracyn (for fungal and gram-positive bacterial infections) and Maracyn II (for gram-negative bacterial infections). Unfortunately, the room for error with a sick fish is very limited. If you suspect serious disease, move to Maracyn and Maracyn II immediately. These two are each about $10, and can also be used together safely. They will often spare much of the beneficial bacteria, but water chemistry should be monitored for Ammonia and Nitrite spikes. If spikes happen, the associated toxicity can be too much for a diseased fish to handle, and water changes are needed. Redose proportionally after a water change to bring the medicine back to the recommended concentration in the water.