To overcome such problems, feed less (smaller feeder) than needed at first.
Then once they demolish the current food (smaller feeder), add small bits of food (pellets) after until they lose interest and look full. That way you limit how much uneaten food is in the tank.
In addition, i have always gone minimalistic (spelling) with piranha tanks.
Black background, black sand, maybe one centre piece, and that way tracking uneaten food is not a problem.
Then once they demolish the current food (smaller feeder), add small bits of food (pellets) after until they lose interest and look full. That way you limit how much uneaten food is in the tank.
In addition, i have always gone minimalistic (spelling) with piranha tanks.
Black background, black sand, maybe one centre piece, and that way tracking uneaten food is not a problem.