Piranha are illegal in Utah.
Title 16, Chapter 16-100
Better check on them to be safe. One city in Utah has fines of $1,000 a day. Check www.opefe.com
for more info.
But to answer your question about cleaning the tank...I've had red bellies completely ignore me, with free-swimming fry in the tank. The next time I went after eggs, both the male and female attacked. Never trust them completely. Any animal will attack if cornered and scared. You don't want to change your username to Stumpy!