The water runs over the top of the sand bed from a t off my main return and flows back into the sump via gravity. You can plumb it any way you like as long as you keep a continuous flow over the top of the sand bed. I didn't need to seed the sand. Nature will take it's course in time. No light according to what I've read. I'm keeping it simple as Mr. Calfo describes it. I've tried mangroves in a DSB fuge on another tank. The problem with that is the mangroves have overgrown the space. cheers!