It's called a "cryptic zone", as promoted by Steve Tyree . The concept is that sponges, tunicates, and other life forms that don't thrive in our highly lighted tanks can grow and purify the water.
Here's a link to his website,
DE Publishing
A search for cryptic zone brings up a lot of information. I've read the book and tried it on a limited basis. As an addition to typical techniques it works imo. The number of pods and tubeworms in my marginal cryptic zone alone justifies it.