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.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.