"map" wrote in message

My plan was to treat the current fuel with enzymes, burn 10 or 20
gallons, then fill the tanks and change filters.

Pump out from the very bottom of the tank beofre doing anything else. First,
you'll need some access to the inside of the tank. Even a dipstick hole
could be enough, or remove a pipe outlet.

Oncce you start pumping you'll soon know what's hiding down there. I use a
longish bit of copper tube attached to the pump with an angled cut at the
bottom, and pump into a transparent plastic bottle (big one). The angled cut
allows the pipe to rest on the bottom of the tank without blocking - this
system gets much deeper than the fuel take-off. Most likely you'll find some
water, and at the interface with the diesel, black slimy bits. If you're
lucky, it's water only.

Empty all the water and as much diesel and slime as you can. Then rustle
around the bottom of the tank with any sort of brush you can get in to stir
gunge off the tank bottom. Pump again. Keep repeating til the diesel runs
clean.

In extreme cases, when there's really too much gunge scraping off the
bottom, you may have to consider steam cleaning. At that stage you need good
access to the interior of the tank. With luck, you won't have to go that
far.

Only then consider the enzyme/antibiotic/any other treatment.

If you have any doubts about how well you've done the cleaning job, either
fit a parallel fuel filter so you can quickly change when one blocks off, or
have a pressure drop warning across your filter to tell you when to change .
.. .

Best of luck. Keep us posted!

JimB
http://www.jimbaerselman.f2s.com/
Compares the cruising areas of Europe, with detail on Greece and N Spain