I've kept piranha - some red-bellies - in an in-store 180g show tank, back in the late '70s. They got biggish, ~8", before we lost them due to some bad additives (blackwater was contaminated). We had wholesale feeders, so we never counted the cost, and they were a constant fascination to the shoppers. Incidentally, they were probably illegal at the time...
They are not a beginner's fish, IMO, and grow much, much too large for your current tank (although they're cute when small). I didn't find them too aggressive, and deduced that we may have had too small a school at 8 individuals to promote "frenzy" when we fed. They never bothered a hand in the tank, and sometimes were quite lazy about getting around to their goldfish. Maybe we overfed them?
Matt
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