Well I imagine you could use it to breed guppies, but they'd quickly outgrow the tank. I used to put my baby guppy fry into my eclipse 3 gallon and move them to other tanks or give them away once they outgrew the tank. I just put a piece of nylon stocking over the end of the filter to prevent them from getting sucked up.
I suggested male guppies because it would be just them in the tank, very pretty. If you do want to breed the guppies, be sure to get 2 females to 1 male, to prevent sexual harrassment of the females!