We have a Betta fish and owned a few goldfishes and the type of gravel we prefer is the multi-colored one. It really brings out the colors of the fish! As for sand, I never tried that and wouldn't either because I imagine it would make the water cloudy. Not even sure if it would be healthy for the fish. Gravel that you purchase at a pet store (or even Wal-Mart) has been treated especially for aquariums. Good luck!