White vinegar is very good to use when disinfecting tanks because it does kill bacteria. It will also clean hard water stains. You can also use a Mr clean sponge; the original version doesn't have any cleaners in it, for any stubborn spots on the glass like dried on algae or any stubborn hard water stains. If you're comfortable with a razor you can always use that to scrape away any built on gunk on the glass. Don't forget the scrub under the rim as well, you may be surprised to see how dirty it is. You can go ahead and rinse the tank out when you're done with the vinegar, or you can wipe it down with some wet towels. The little bit of vinegar that's left over won't do anything to the fish. I've cleaned up many used tanks this way and haven't had any issues like fish death or plant growth problems.