i want to get some piranha's, and this will be my first time... I only have a 30 gallon tank and I would prefer some red bellies because after checking the prices, they seem the most affordable. I want to use the tank i have, should i just get some little pirahnas, or less piranhas but bigger ones. How big can i go? how many can i get? I realize this is a relatively small tank... any suggestions??