I'm new to the world of dsb's and can't really help ya here. The only dark spots I've had were do to algae growing in the sand, against the glass. I'm still getting bubbles in the top inch of my sand and it's been 3 months since I built my sand bed from 2" to 4". Read all of the info provided by people on this board and do some of your own research on the net. Take all the info, shake it up and form a plan of your own, something that sounds right to you.