Awesome thanks for the link! I remember hearing about his crypic zone concept at one point and I think watching a video showing one of his crypic stock tanks, but hadn't been able to find it. I'm going to order "The Porifera" now. I've been reading "The Biology of Coral Reefs" by Sheppard, Davy, Pilling, and Graham which has a short, but interesting section on sponges (though doesn't cover aquaria). It talks about some species being able to filter out cyanobacteria and even viruses -- as well as removing as much as 93% of all particulate matter in the water column. I assume that means at some point in a reef tank too many sponges would compete with corals for food, though.