How to clean large home aquariums?

I'm talking like BIG home aquariums. Like where you have an entire glass wall with water and fish behind it you know? do they have like a hatch above the aquarium on the floor above that they get down inside of it to clean it. or a really good filter system, or cleaning fish. I can't figure out how to clean them. so if you can help me out with this you'd be much appreciated!