Well I SINCERELY hope he doesn't plan on putting 7 piranhas in a 20 gallon tank.
Damian, if that really is the size tank you're keeping your fish in, have you done ANY research on keeping red bellies at all? I'm no expert, I've never kept them (and never will), but even I've read up on them.
They grow to a foot long. That's half the size of the tank you described. They also eat messily and foul the water they're in very quickly, and therefore need an extremely efficient filtration system. The LAST thing you needed was 5 more. What are you actually feeding them and how often?
Just looking at that other thread - 2 oscars, a green terror, 2 plecos, 2 convicts, in a 20 gallon tank? I'm actually speechless. Out of interest are any of these fish still alive considering that thread was from april and you completely ignored the advice of the people trying to help you?
Now I apologise if I appear to be being rude, but it well and truly grinds my gears when someone keeps fish that require advanced care they're not in the slightest experienced to keep. It's just the epitomy of irresponsible aquarium husbandry.