Why not a trio of gymnogeophagus. like anything else when it’s 100 in august will want some kind of shade- but they can stay out all year and you can harvest fry in the summer to watch grow. Give them some hornwort and water hyacinth for shade and cover. Put a couple few red swordtails or plates in there so you have something you can watch. Or put sane on the bottom so the fish are somewhat visible.