Re: Cleaning a fuel tank
Unless you can get access into it, you will need to take it to someone. Diesel tanks seem to grow a film of slime on the inside which is very difficult to remove just by sloshing stuff around.
The best case is that you can physically get in there and scrub it. This usually requires an access plate but sometime removing the fuel sender will grant access. Keep in mind that there are baffles which make it hard to get everywhere.
A powerful jet of something tends to work pretty well too. I have steam cleaned them before with good success but I would imagine any old high pressure jet would work (not garden hose jet, they are not high pressure).
Radiator shops will sometimes be willing to clean one and they can do a decent job. They have things like ultrasonic cleaners etc that can break stuff free in places no person can get to.