filtration?
1- i would stop with feeders (or reduce to once a month)
2- swich them to frozen like talapia,shrimp,cod ,pollock... also pellets
3- make a water change schedule so you can follow it and not miss one (should do one per week at least) eventually it will just become habit)
4- upgrade filtration (even if they filter says up to 75 gal for a 55 gal pirahas are messy so they could easily use 2 of the filters or a large cannister
5- powerhead for activity which will help them to be hungrier and also grow more
5 little things that will help them grow bigger faster
also a larger tank would be good too