Hm.. I bet those deposits are so hard to get off because it was used as a fish tank before. Aquarium salts/conditioners have some strange things in them... I dread the day when I have to re-purpose my fish tank. It's never that hard to clean deposits off my frog's tank using vinegar and water, and the sides of that get sprayed with hard water about 5 times a day.
Luckily for me, Houdini can't see and is therefore not offended that I put him in a 30 gal. fish tank with the world's most persistent residue

My tank is the same type as yours, so to give extra space, I fixed some globs of GreatStuff foam to the side with aquarium silicone (cocofiber pressed into them to make it look natural) and use that as a sturdy anchor point to add fake vines and such. It's strong enough that he can actually climb up in the vines. Also, if I want to change things up a bit, I can just pluck out the fake ivy and stick another fake plant in there. The only problem is that it's semi-permanent, so you have to make sure you want it wherever you put it (lots of hard work and an ice-scraper if you don't), and it gives them access to the cage-top, so you have to be extra paranoid about securing the cage.
Just thought I'd mention that, since my snake became a lot more active after I added that to his tank.