20 gal tank - good mix of community fish ?

Actually before you switch the substrate, you might want to try other things first. It is a lot of work and a big decision indeed. Even after you have bought the new substrate, it might not guarantee to fix the problem. Nutrients, lights, CO2, every one of them is important.

As for now, the fish is your priority. Get them to act normal first before doing something dramatic with your aquarium.