I've heard that you can use it as a general rule for fish under 7 inches. I don't know if that's accurate. You could do a Betta tank with some inverts. Maybe some shrimp(Ghost, Cherry, Amano etc.) or a couple ADF
's, or snails. Nerites are a good choice, you could do:
2 Nerites or
5-6 Shrimp or
Maybe 1 Nerite and some Shrimp.
Or 1 Nerite and an ADF