Prime's main function is as a dechlorinator. It binds ammonia into ammonium for up to 48hrs. It does nothing for nitrate. Your nitrate is high from only doing a 25% weekly water change. A 10in cichlid in a 55 will produce a lot of ammonia, and your functioning biological filtration converts the ammonia to nitrite, then nitrate. Nitrate is removed via water changes. Up your w/c to at least 2x weekly of 50% or better per change, and your water quality will greatly improve, and FH will be much happier, and appetite should return. Clean water works wonders.
