20+ Aquariums is just way too much! Your looking at an entire room for all of them.... Or one corner of a room with 2 racks..... Racks are are just better for the keeper and the animals. They are less clear "less stress for the animal" they are easier to clean and like you mentioned they hold heat and humidity better. I'd tell your friend to back off and do some research. Balls spend 80-90% of their lives in rodent holes cramped up, and sitting rodent feces.... They are more than happy with a nice, clean, and warm tub!