There are some small sunfish, many minnow species, several beautiful darters if you have lots of current. Try doing some collecting and seeing what you have in your area
I think I've said it quite a few times in other threads, but it is illegal to transport live fish away from California waters so be careful in what you do.
Goldfish are not N.A. natives.
For a 20g look into killifish, pygmy sunfish, madtoms, darters, smaller minnows and Enneacanthus sunfishes. You probably won't find most of them in the wild in CA but there are a few online places you can purchase them.