FRESH WATER FISH

This information from IWProductions (bettacare.blogspot.com)

Before you panic and start dousing the tank with every chemical you can find, let's take a look to make sure there's something wrong.

Here are some characteristics to look for in a healthy betta:
-High energy. Most bettas are active much of the time. They do take breaks, but will move around quite a bit, especially if someone comes near them.
-Big appetite. Bettas are big eaters. If you keep feeding them, they'll likely keep eating. Not that you should overfeed your betta, but their appetite is a good barometer to their health and mode.
-A healthy, clean looking body. Bettas have a fairly smooth and uniform body. Their scales should look neat and orderly and their fins should be whole and spread out.
-A bright, brilliant color. This is especially important for male bettas. They should be colorful. Bettas that are dull and drab or look "washed out" may not be doing well.
-Healthy, normal looking eyes or gills. The betta's eyes should be uniform and proportional and the gills should look healthy and smooth. The gills should close well against the betta's side.

Here are some characteristics to look out for:

-Low energy. A betta that sits at the bottom or in a top corner of the tank and mopes around might not be doing well. If this behavior persists then there may be something going on.
-Not eating. Bettas like to eat, so be alert for changes in appetite. If your betta goes on a hunger fast or ignores food then probably it's ill.
-Any strange marks on the body. If your betta's scales are ruffled up, you see sores, or there are any strange growths on its fins or body then that's a pretty clear sign that it's sick.
-Losing color. A betta that's dull and has lost its luster is not doing well. Either something is going on with the betta or its food.
-Look out for swollen gills or protruding eyes. If a betta has a gill disorder or one eye starts bulging that's a sign of some serious and common problems.

If a betta has a gill disorder or one eye starts bulging that's a sign of some serious and common problems.

Keeping an eye out for any changes in your betta's appearance, behavior, or mood and you're a long way towards maintaining a happy and healthy fish. At the first sign of these symptoms it's best to make a water change and isolate the betta.