Vinegar is pretty safe. Although you would need to tear down the tank to clean the insides. <Remove fish & everything else to a clean container, soak buildup with undiluted vinegar, perhaps using vinegar soaked rags, then scrub clean - use a razor on the stubborn bits. Then all you need is a good rinsing & the tank can be set up again.>
If it is outside, you can clean with the tank set up, just try not to get vinegar inside the tank, as too much will drop your tank pH.