Piranha's are one of the hogs of the fish world. Most piranha tanks need twice as much filtration as normally stocked tanks do. They are very messy eaters and they like to eat alot.
You'd be shocked if you were to measure nitrates, I dare to suggest, judging by your macroscopic observations.... Mine spiked to 200 ppm in the past ... :-O. Not good.
Rule of thumb for RBPs.... like FormulatedFire said twice as much filtration as normal... so 55 gallons should have at least filtration for a 110... and also when piranhas are young under 5" they can be in a smaller tank but when they get bigger they need 20 gallons of water per fish... so 4 red bellies 75g or more... do you have any other fish in with them?