<fuel tank observations from diesel snipped>

try a brief cleaning with various solvents to see how easy it will be 
to clean up the diesel tank.  if it goes rough, use the gasoline one. 
 the tarry substance is horrid looking but most likely a microbial 
flock from bugs that thrive in fuel oils.  they setup residence at 
the interface between water globules and the fuel, utilizing fuel as 
their carbohydrate source.

feel those big flakes/sheets of crud.  if they are gritty or sharp 
they contain metal and you should switch tanks.  if they are soft 
gooey and just generally a disgusting slime, then the physical 
integrity of the tank is likely okay.

and we won't even MENTION what grows in ice machines!

-dan