Vinegar or Bleach will do a good job. If you use bleach, mix 1 part bleach to 10 parts water, then when you're done, soak the tank in water and add 5x the recommended dose of de-chlor. Razor blades will also probably help you out. If the seals look good, leak test it before going through all the trouble of cleaning. If they look cracked or shrunken, go ahead and re-seal it. It takes a couple hours, but will only cost probably $10 if that in supplies.