At least 6 inches and like wolf said do a lot of water changes and they will grow quicker. Red bellies do not apply to that 20g per fish rule. I have 14 reds and 4 ternetzis in a 200g and they live just fine. Crowding reds is not a bad idea either since it seems to tone down their aggression amoung each other. They are not allowed to make territory because of all the other fish in the tank. So they just deal with each other. This has work for me countless of times. Good Luck.