First, the enclosed growth compartment increases the dwell time of the water and bubbles inside the unit so that they rub the algae many times before leaving the compartment. This actually reduces nutrients (nitrate, phosphate) to a level lower than the outside water, which allows greener growth to occur sooner (lower nutrients grows greener algae) inside the unit, especially in high-nutrient aquarium water. In other words, the SURF2 creates a lower-nutrient ecosystem inside itself which is different from the rest of the water in your aquarium.