Don't spray Muratic until/unless you water it down. It is pure hydrochloric acid which is a very effective and safe cleaner for calcium based stuff, but it is pretty nasty at full strength. Don't breathe it and don't get it on you, especially your eyes or other sensitive tissue - otherwise, it is cool. Always pour acid into water, not the other way around.
I might mix like a 3G of water in a bucket from the hose with 1/3 gallon of muratic. Then put the tank outside on a flat surface with the back down - on some rocks on some 2x4s would be good. Pour in a bit of your mixture to where it barely covers the back glass. It should bubble and you can probably easily scrape it in a minute or two. Then, turn the tank and do the other sides and bottom. This will be more potent that filling the tank all the way and diluting the acid with 90G of water. Just rinse it with a hose when you are done and it will dilute down to a un harmful PH in no time.
I use it all the time and it is no big deal... just be careful. Rubber gloves are a good idea. If you get any on you just wash it off really fast. If you are really a sloppy kind of person, then sunglasses are a good idea to protect your eyes.
It is safer than vinegar for your tank if you don't get it washed all the way - it will just dilute down to nothing whereas the carbon in the vinegar can fuel a bacteria bloom.
