A couple of things...
Please elaborate on the word "room"?? are you implying you want to give this animal a room in your house??
Secondly, You do realize that virtually all monitor species are burrowing animals and need deep dirt to dig tunnels in.
taking it out then plopping it in a tub is stressing this lizard out more than it already is.
See if you can get a veterinarian to sell you some Normosol R electrolyte and a needle. Then what you want to do is take a small (thawed out F/T) mouse and inject it with the electrolyte solution until it swells up like a ball, then offer it either in a dish or from a set of tongs.
Basking temperatures, 130+ degrees, ambient temps @ 85 or so.
As the name "Water Monitor" implies, you should have a large water source, such as a cat litter box filled with water inside the enclosure that your lizard can get into at it's own leisure.
This has to go.. It simply will not do, and the animal's condition will continue to deteriorate rapidly until you get rid of the fish tank.
Thank you.