So I have 2 cory cats in a 30 gallon tank, the tank is planted (not as heavy as i would like). Anywho, I also have 5 guppies in there. My concern lies with one of the cats. One of the two cats, is fairly large and active all the time. The second one on the other hand is about half the size of the first. He kinda just sits there and doesn't really eat when food is down there. The large cat swims into the smaller one and it kinda just brushes it off and sits there.. What's wrong with this cat?