Water to take off all the loose stuff. If there is hard water buildup, then follow with pure vinegar ... might have to let it soak for a while for really hard deposits. <Soak paper towels with vinegar & lay on surface for vertical surfaces.> For really tough scales, you might have to resort to CLR.
Rinse well after all the vinegar/CLR treatment. Let dry to sterilize. <Some people use bleach, but for a tank that has not housed diseased fish, letting it dry is prob. enough.>
While you are at it, you might want to check the seams. If any silicone is loose, now is a good time to repair it. And test for leaks before bringing into the house!
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.