I use black sand (3m colorquartz sand) and it seems to make my bichirs greener. I really like it. Many people say that a red or dark brown color sand makes the colors on a bichir pop out the most though. See some of Hao's old picture threads to see the good colors on his polys. Also, some people say that having black or dark substrate makes certain bichirs like endlies a lot darker, but I like really dark looking endlies so I don't mind.