i currently have five in a 72 gal. and that's definitely pushing it. luckily there's lotsa plants in there where they can hide. i 'm going to say three for life (reds and other pygos can get BIG, FAST!). i bought a 5.5" caribe about a month ago, and i swear if you sit there for a few minutes you can see him growing. he's already outgrown a red his age that was bigger than him when i got the caribe- and that red was the fastest grower before that. you get the idea. any pygo will grow like a weed. you could probably push four with no problem though. i'm guaranteeing you three with no problem ever, unless you just get a crazy fish, then no amount of gallons per fish will help until you get into the 100+ gal. size and that's way off your original question.