look dude I mean no disrespect but if u go online on wikipedia I've been reading.it tells us how to differentiate the sex and I also read on the internet that 5 are perfect for a 100 gallon aquarium. So one makes a it perfect for a 20gallon. You can go and check at least that's what I read.
#1 - Wikipedia is not a sufficient source. People in the P section have done their homework, have had years and years of experience, and are much more knowledgeable than you think.
#2 - Look up the term Sexually dimorphic. Then look up the terms 'Piranha" and 'Sexually Dimorphic' in sequence.
#3 - Do you think a 7 to 10 inch fish will do well in a 20 gallon? Have you ever seen what that looks like?
- No disrespect, but give us some credit instead of already knowing everything. These people can teach you a thing or two.