I definitely wouldn't put a male and a female, as the female can get very stressed out by the male trying to mate with her, and she won't have much room to get away in that size tank. A Betta, as mentioned, is a possibility. Your best bet, in my opinion, is just keeping it for the fry, even if it does look empty, because that's what's going to be healthiest. If you add another fish in with fry, you have more waste and ammonia, which means more water changes to keep the water parameters good for fry.