well, in the 15g i would get a chameleon! love those guys. then i think i would put gerbils in one with the top cage bit, and 12 tanks for fish. on a set of steel shelves, fit as many side by side as possible. 1 halfmoon betta on each shelf down one side, maybe 3-4 tanks depending on number of shelves. then some different guppies, all going down in a line, definitely some of the more fancy ones. then also going down in a line, albino corydoras, bristlenose pleco's, red honey gourami, and quite a few different tetras. and a couple of big external filters under the shelves to run the whole lot.