I've dusted off a few old aquariums and what I typically do is clean them with vinegar and then reseal them. A vinegar soaked sponge will get a good amount of the build up off and to get the last bit I lay the tank on its side and fill it with vinegar (so the vinegar pools on one side panel), leave it set in the garage for a few days, and repeat 3 more times to get every side. Sometimes the build up is really bad and etches the glass a little though.