Hi everybody.

Just found this site and was interested in all the ways to clean gas tanks.
My brother and I have a radiator shop and a salvage yard and we have occations to clean gas tanks and what works best for us is hot water and the granulated automatic dishwashing soap. The soap is some what caustic so it cleans organic build up from gas tanks. The dark brown goo and crud one sees in an old tank that has been setting around for a long time with old gas in it.
The procedure we follow is: the tank is emptyed and flushed out with hot water, a steam cleaner works best but the car wash will do nicely. Next buy a small box of dishwasher soap and add it to the tank and fill about 3/4 ths full of hot tap water and agitat until the soap is disolved. Then let it set overnight, empty and flush again. the inside of the tank should now be pretty clean, if not repeat the procedure. When you are satisfied that the tank is clean obtain a quart of muratic acid at the hardware store. Follow the precautions on the lablefor breathing the fumes, (not a good thing to do). Pour the acid into the tank and slosh around until any remaining rust is dissolved. Rince the tank again and dry completely. That is how we clean gas tanks and it works well for us.
Good luck and be careful.