Block up one wheel of your tractor or other appropriate vehicle. Strap tank securly to wheel and add whatever (gravel, old rusty nuts, bolts, etc.). Start and let tractor idle in first gear for the afternoon. Results, clean tank and nuts and bolts look pretty good too. Just remember, if tractor falls off block, there is no stopping it.
That is a similar technique most companies use to clean chemical tanks. It works like a rock polisher. Fill it with a quarter tank of pea gravel and get it to spin some way (the tractor will work) and if you leave it spinning long enough it should clean around baffles as well.
