People say that they will become nitrate sinks, I had one many years ago on a tank that was algae free and everything was huge and open. All the test I did on it were in the 0s. I have one that I have never used and thought about it a couple of times but to many things hanging in the tank as it is without adding another PH. It also I think depends on how much bio-mass you will have in the tank as well. I know a few LFS that use them on there fish sections just because it does remove a lot of ammonia and nitrites in these sections the fluctuate with the bio-mass......If it is on a frag tank with low bio-mass other then corals you shouldn't have an issue, they due serve their purpose if used properly...