[boatpoker]

There is some amount of water sitting in the bottom of pretty much all diesel tanks. If you can't get it out more frequent filter changes until it disolves may be the order of the day.

 

[boatpoker]

If the tank is aluminum there may be some interaction between the two different metals with the softer aluminum scarifying itself before the file? As mentioned by Boatpoker, water may be present, speeding up the process? Maybe some chemical expert could comment?

Excellent point on the aluminum, I should have thought of that. Galvanic corrosion could happen quite quickly if that is the case.