most puffers are best kept in a species only tank, though some have had luck mixing them. in a 20 gal you could have a nice little group of dwarf puffers (4-8 maybe), but they only get one inch long.............. they are fun and active fish, but not really "showy".......
if you want a bigger fw puffer, South American (C. assellus (sp?)) get about 3 inches long, and are relatively peaceful. you could have 2, maybe 3 of these in there. of any puffers, these are the only ones i'd chance mixing with some other fish............. but be prepared to move fish out if you do.........
or, if youre ready to tackle bw, you could do a couple of figure 8 puffers......
keep in mind, most puffers aren't "showy" fish, they hide, blend in, you often have to look for them (unless of course its feeding time or soemthing foreign, like the gravel vac, is in the tank............
). they are great fish, but chances are, they won't catch someone's eye when they enter the room... the tank might, but they'd have to look for the fish depending on what type you got....... of course, the lack of immediate fish in view is what draws most people to my puffer tank, to see whats in there.......... your choice
