Scraping is certainly the fastest way. Should take about 5 minutes with a heavily encrusted tank. To use a chemical (vinegar, HCL), you'd have to fill the tank and soak it for a day, or less depending on the strength of the acid. If you use HCL, you need to neutralize it before disposing of it.
You should be thinking now," gee, acid sounds like a lot of work. Ill just use the razor!!"
