there's no reason why you can't have fish with dart frogs. as far as the amount of water, 5 gallons would be the minimum, and you would be looking at micro fish like chili rasboras, or one of it's cousins. for larger volumes of water, you could go with endler's live bearers (a form of guppy from south america). plecos are fine, as long as you go with the small species like bristlenose or rubber lips. (4"). otocinclus would also be happy in a small volume of water. just remember, please, to keep the health and welfare of the fish on the same level as the frogs. so don't go trying to cram something like a gourami into 3 gallons of water volume (i've seen it done)