Welcome to the forum.

For all aquarium fish to prosper, you need to make sure that the water quality is of excellent condition. Your aquarium water should be free of ammonia and nitrite and low value of nitrate.

Ammonia and nitrite can be maintained at zero level if your under gravel filter is able to cope with the fish waste. Nitrate on the other hand can be removed with regular water change (weekly at 20%).

Cheers