Also, home depot and lowes will sell it, and many hardware chains (i.e. True Value, etc.). Look in the paint section - they sell it to clean concrete floors prior to painting.
It works really well and is safe for glass and silicone, but it will destroy metal, painted surfaces, plant life, etc. and it can burn your skin (in a way that's moderately irritating, not REALLY bad). It smells kinda bad and the odor can be irritating. You definitely have to respect it, but it's SO effective for stuff like this it's worth the effort IMHO.
I'd start by getting a spare milk jug or other sealable container, and dilute 2:1 water:acid. Then put the tank on the ground (driveway, other area you don't care about) and pour a cup or two in. Let it sit for half an hour or more, then rinse with a hose. Turn the tank so another panel is down, and repeat. If the deposits are REALLY stubborn you might have to scrub for a minute or two but usually they'll just rinse away with water. For stubborn deposits I get a soft bristle brush that can be screwed on to a broom handle, so I can scrub from a few feet away. I've had deposits so bad they wouldn't budge with vinegar or other methods after hours of effort - this method took them off with a few minutes of scrubbing.