In my 30 gallon reef, I use an HOB filter with activated carbon that I change about once a month or so. I noticed that my water is cleaner when I do use it. And as far as restricting coral growth...tell that to my star polyps that have tripled in size since I bought them in May and my button polyps which have literally sprouted onto 3 new rocks in 2 weeks and my mushrooms that are twice the size of what I bought back in March! I bought a colony of 5 mushrooms, and I now have almost 15 that are twice the diameter as the 5 original ones.