Ok I just have to step in it. I keep dwarfpuffers and have for several years now. I started out with 4 in a heavily planted 10 gal--they wwere the size of small jellybeans (juvvies) and they were all friends, hung around together, ate together, etc. Then, when they started to mature, one male got hostile toward the other fish--kept the chasing the female who was terrified of him, and kept the other two (who turned out to be males--they can tell a lot sooner than I can) cornered in a tight corner at all times--they might as well have been chained to that corner becuase he would not let either of them wander out, even to eat. He nipped their fins and just made their lives hell. The female didn't like him but he was trying to force himself on her (she liked the other two). So, I separated them--smallest male went into a barebottom 5 gal. tank with java moss and lots of floating plants; put in a tank divider in the 10 gal. and 1 male on each side of the divider. This is the way they have lived for the past 6 months. The aggressive male made it impossible for the other fish to live with him so now he's all alone (and obviously lonely, hides a lot now). The little male living with the female still doesn't seem to know they are supposed to be breeding (they're just good friends, go shopping together, met for drinks after work) so this morning I switched him with the other non-aggressive male who immediately started wooing the female and she didn't seem to be resisting much.
So, I do and have kept 2 dps in a 5 gal. tank and they are healthy and happy. All my tanks get a 50%-60% water change every week and a thorough vacuuming--any tank with any leftover food gets vacuumed immediately. The last old oto died last week so at the moment there aren't any other critters in their tanks (snails get eaten immediately, I'm hoping to put a couple Amano shrimp into the 10 gal tank soon). They used to have a bunch of the non-red Red Cherry shrimp in with them to keep the algae down but when the males went into their hormone induced mating mode, they killed them all within an hour.
Rules of thumb--keep an eye on your fish and if you see any aggression, DO SOMETHING fast because it's not just a friendly spat but will get worse if you leave them all together. If you use a smaller than about 5 gals per dp tank, you MUST do cleanings religiously and test the tank regularly so the ammonia and nitrite levels stay at zero at all times (no shurking your duty to the fish ever). If you want to add a new fish, first quarantine it for at minimum a week (better would be 2 weeks) to make sure it won't bring some bad thing into your established tank (I quarantine in the tank the fish will be living in eventually by partially submerging a smaller tank into the big one--fish get to see each other), then before releaseing the new fish, rearrange everything in the established tank so any fish who has staked a claim on some territory has nothing to defend--and keep an eye on everyone (the newbie could turn out to be hatefully aggressive). And be ready to separate fish into other tanks always.
