They have box with holes in them that are supposed to serve as a fuge inside the tank. I like the hang on the back fuge, myself. I have a refuge in a 20 gallon below my 29. I have some microfauna and the cleaner clams with sand bed and rock rubble. But the hang on the back are good for limited space, and the inside the tank are good if you are really limited on space. I have used a plexi glass tank (with the holes) to introduce tank mates that I wasn't sure how the process was going to go.