1) Citric acid isn't that expensive, and it's available on Amazon, or get a pound from my bag for $5. Put the tank in an orientation where you can puddle water on the surface you want to clean, pour in a half pound of citric acid, and then wipe away the calcium after a day or so. Not much of a safety risk, no fume risk, and if it's calcium based it'll take it off. Won't take anywhere near a half pound, but the stuff is really cheap so why not? Cleans up with soap and water (or really just water).
2) Don't use CLR, that stuff fogs glass (guess how I know).
3) BTW, don't leave anything you don't want rusted within 10' of the HCl while you have it open, that stuff travels and rust randomly appears for the next while. Fortunately I got to watch someone else make that mistake instead of doing it myself.
4) If it really is etched you *might* be able to buff it out with cerium oxide or you might be able to locate one of the more exotic 3M discs designed for glass work. I'd offer up one of mine but they're all in a box half way across the country. Don't overheat the glass.
5) As someone who's spent several dozen hours getting rather serious scratches out of both glass and acrylic aquariums if you do come across a decent scratch you want out of a glass tank don't bother. For the time it takes, unless it's on a full SPS reef you don't want to tear down, you're better off getting a minimum wage job and saving up for a new tank. It'll take less time than trying to learn to get a decently deep scratch out of a glass tank, and then doing the work to get it looking flawless.
