i use one. i have the tank running for 9 moths. no nitrates or phosphates to speak of. i barely run gfo in a 57 gallon tank, i have 3 antheis, 2 clowns, a fairy wrasse, a doty back and a kole tang. i feed 3 times a day and my corals 2 times a week. i like the natural look and think it provides a benefit. i do have 2 fighting couch and a cumcumber to help with the sand bed and maybe 25 nass snails. maybe in 4 years when my tank crashes i will feel differently, but its been working for me