NOT BLACK. Black gravel will make your flower horn look terrible. It makes many flower horn look like charcoal in color. Light colors are good. I like river stone, its got a great natural look and the light colors help flower horns look their best. I would also stay away from any kind of sand, Flowerhorns are very aggressive and like to move things around. Sand can get kicked up into the water and then sucked into your filters where it can tear apart and destroy your magnet drive motors found in most filters and the impellers.