if it only climbs to 20 degrees in summer you could go the route of hardy tough fish, you will have trouble with just about all inverts though. but many triplefins, Favinogobius, Olive rockfish etc.... would do ok, much above 20 degrees and you will have trouble, thats what forced me to close down my 20L mudflat tank, new house, un insulated garage, summer.... then again, I had a heap of clams in there that did well for over a year until that happened