no questions a stupid one!
it depends what you mean by cleans?
if you mean filter out fish waste produce. This is done by a few things. Ammonia is converted into things less harmful to fish, corals inverts etc by the bacteria in the live rock. The powerheads (pumps) are there to circulate the water over the surface a1rea of the rocks meaning the bacteria inside the rock has a constant food supply. They also break the surface of the tank allowing oxygen into the water.
things like solid waste and algaes are usually consumed by our clean up crews, them munching keeps the tank clean, they are essentially living on what could turn into polloutants.
Protein skimmers, they skim protein. Dissolved organics in the water collum. The natural reef is a very low neutrient environment. We are trying to replicate this, giving our stock as close to natural surroundings as possible. You cant really see the dissolved proteins, but when you have seen and smelt the mess that comes out of a skimmer, you will know that they are imo an essential part of our tank.