I have four female convicts in a 20 gallon tank. Fluval power filter. They used to eat 8 pellets twice a day (16 pellets a day), now they MAYBE eat 8 a day. This has been a gradual decline in the amount they eat for maybe two months now. I tested the water and nothing has changed from how it's always been. They are still as active as they have always been the only thing that changed was they stopped eating as much and I don't know what to do? Please help!