Sounds pretty simple, doesn't it? Prevent disease, and you don't have to worry about your fish getting sick. But if it were that simple, then there wouldn't be entire sections of local fish stores with nothing but medications, right? Wrong. It IS simple. Hopefully this will help you to have healthy fish.
The first and most obvious thing is don't buy sick fish in the first place. Take the time to observe the fish in the dealer's tanks before you buy them. Don't let a clerk rush you, take your time. Here is what to look for:
Finally, especially if you are purchasing fish like Discus, make sure they are eating. Simply ask the people there to feed the fish. If they refuse, or say the fish just ate, do NOT buy it. A healthy fish WILL eat even if it was fed just a few minutes ago.
OK, so you did all this and now have your new fish in hand. Now you just go home, acclimate them to them main tank and plop them in, right? Nope. Just because the fish looked and acted healthy at the store does not mean that they aren't carrying something that just hasn't cropped up yet. All new fish should be quarantined before being put in your main tank. A QT tank need not be anything fancy. For smaller fish, a 10-gallon will work fine. For medium sized fish, I like to use a 29 or 30 long. For whoppers like Oscars, you are going to need something along the lines of a 55.
The equipment in a QT tank needs be only the basics. An appropriately sized sponge filter, preferably seeded in an existing tank, heater and light. It's also a good idea to fill the tank with water from the main tank the fish will be going in after QT. Simply care for the fish in QT the same as you would any other tank, and observe him for any signs of problems. If anything develops, treat appropriately. The QT period should be a bare minimum of 2 weeks, 4 weeks being the best. Once you have done all this, there should be no problems, right? Again, nope. Prevention does not end there. Proper maintenance and monitoring of water conditions in the main tank is a must. If you don't do regular water changes, preferably 30-50% weekly, the water conditions will deteriorate and weaken the fish's immune systems. Obviously this will lead to disease.
Proper water parameters for the fish in question is also an important factor. A soft water fish just is not going to do well in the long run in hard water. I don't care if the fish was raised in the wrong water or not, it is going to do best in the water type it spent millions of years adapting to. So keep your Lake Tanganyikins in pH 8 and your Discus in pH 6. Not the other way around. Proper feeding is another factor. A bad diet is going to affect fish just like it would you or I. So a varied, nutritious diet is a must. But it must also be an appropriate one. For instance, if you feed your Tropheus a meaty diet, you WILL end up with sick and dieing fish. They are primarily vegetarians. So be aware of the needs of your fish.
This brings us to the final point. Everything I have said hinges on one simple factor. You HAVE to know what your fish need. This varies from fish to fish, even within the same families sometimes. So you MUST do good research BEFORE you buy your fish. Without knowing the fish, you won't know what normal behavior should be. Or what water parameters it needs. Or what types of foods to feed it. Once you know what your fish needs, preventing them from getting sick is as easy as pie.