Neons will need filtration, goldfish will muck it up, with no pump or filter you will be restricted to paradise fish, bettas, maybe some of the smaller gouramis, maybe a dojo loach or two, and mosquito fish, or you could try 1 ricepaddy eel but I am afraid it would take a hike come first rain, definitely add some plants like elodea or cabomba. Another consideration is overflow from rains, you could drill some 1/2" holes below the lip and glue plastic screening across them so the excess water would flow out without the fish going with them. In Louisiana I had a coulpe 1/2 barrels outside that contained plants, mosquito fish, and pygmy sunfish.