What are the fish physically doing? Watch the fish, how they are acting gives you clues too.
Based on the info you provided, I would suggest changing out about 20-30% of their water for fresh water that has had the chlorine removed.
Most likely this is a water paramter issue. If they have been eating like crazy, it also means they are pooping like crazy. What kind of cats do you have? Some are more aggressive than others.
A water change, not all the water as this will shock them, will temporarily solve your symptoms. BUT it's not a fix. Just a band aid. Give me some more info, and I can get you better answers.
Good luck.