What fish clean the tank?
Catfish and types of snails will eat the stuff in a fish tank
No, it is important NOT to clean the tank with soap! Soap residue will likely kill your fish-- even if you think you've rinsed it thoroughly, there is likely enough soap left on the tank to kill your fish. Clean your tank with hot water, then leave it in the sun to dry if you are concerned it might have contaminants from previous fish that lived (and possible died) in the tank. If the tank…
They can't. You do not have to remove fish to clean a tank, as you should never be cleaning the entire tank; it will remove the good bacteria needed for the health of your fish. If your tank is over stocked and you are unable to upgrade to something bigger, many local fish stores may be willing to give you store credit for returning some fish.
I suggest that you learn more about your fish in your aquarium. One of your fish might be a bottom sucker which means that they eat all of the dirt on the side or bottom of your tank. If one of your fish are bottom suckers, that's probably the reason why your aquarium is clean. If none of your fish are bottom suckers, than somebody must be secretly cleaning your tank for you.