As it grows, it uses nitrate and phosphate. When you trim and toss (or sell) you are exporting it from your tank. I don't think you want to let it get as big as it can - it will start to die off it is gets too dense. The more you have, the more nitrate and phosphate you will suck up, but again, go too much and it will die/rot and actually cause more contamination than it removes.