Cleaning a standard tank is hard enough let alone something as inaccessible as what you posted @J.H. I bet that these tanks either are professionally cleaned or end up as a constant frustration for the aquarists that add them into their home.
I bought a 24 inch X 24 cube aquarium about 7 years ago that I curse every time I clean it. It is a pain in the ---!
I know I am stepping on some toes but I cannot stand bare tanks. Nothing but fish, no substrate, no plants, rocks , wood. Anything. Makes me feel sorry for the fish and science has proven that it mentally stunts the fish