Under gravel filters(UGF)are not very popular in saltwater anymore.Mainly for two reasons,#1-They usually end up trapping excess food and fish waste.This causes excess nitrate readings in the tank.Even with reverse flow,they are more trouble than they are worth..#2-they require using crushed coral(CC) for substrate.CC is also known to contribute to high nitrates.A deepsand bed is preffered these days...