Nice rays! The darker substrate causes them to darken up and become more "cryptic" for lack of a better word. I am a fan of bare bottomed tanks, however, I kept my flower rays on black Estes sand because it brought their color out so vividly. Just watch that the gravel/pebbles are not too coarse...you'll know if the rays' bellies become irritated. Also, make sure you vacuum the substrate regularly to eliminate decaying matter that can cause a change in your water chemistry.