piranha are sexually demorf (excuse my spelling) but i mean there is no way to sex them unless you physically see the female laying eggs and the male fertilizing them. a 30g will be too small to even try and get more then one species of piranha in the serrasamlus species to cohab it is hard enough in a decently large tank and iam not to sure how many people can answer piranha questions on MFK id look more at
www.piranhakeepers.com for piranha questions to be honest