Hunger and Health

The difference between an aquarium fish and a wild fish is that an aquarium fish is stuck in a puddle. Whereas a wild fish has all the fresh water it needs to thrive. Since the water provided to an aquarium fish is limited, it takes an outside force to change the water in order to keep it fresh. If the water is gets bad, and this happens real fast in the absence of filtration, the fish will get sick. A sick fish doesn't move in the normal way and you will notice it.

Hunger

Hunger is one sign of good health in general. This applies not only to fish, but also to other living things. If your fish
is literally jumping out of the water when he sees you, he is hungry. When you observe this type of behavior for a long period of time, you can say that your fish is healthy and happy.

Happy Convicts

I have a tank full of Convict Cichlids. When I approach them each time, they all cluster at one spot and display extreme enthusiasm towards the meal to come. Big Cichlids like Oscars will also display this kind of joy for food when healthy. They are also known to eat from your hand.

Health Issues

If you find that your fish is not eating or perhaps eating just for the heck of it, these are signs of something gone wrong. Normally the fins of a fish like this, will be droopy. So make sure you start your water diagnostics. You can also observe your fish for any signs of disease or infection and treat accordingly.