If there's something in the tank you want to kill, nothing beats bleach. Make sure to wash and dry the tank thoroughly afterwards and let the tank sit for a few days to be safe. Bleach is just chlorine, so it'll disperse just like the chlorine in tap water. Adding a good amount of dechlorinater when you first fill up the tank is a very good idea, though after a few days your tank should be bleach free.
Don't use soap, since that'll kill your fish and there's no way to be sure you've gotten all the soap out.