Go by West Marine and get some Startron tank cleaner. It eats all the algea to the point it will pass through the filters and be burned up in the combustion process. Some folks disagree but a few tank fulls worked for me.

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85 300D 107K present
82 240D 190K present- next-to-slowest car I ever owned
81 240D 1995-2000- slowest car I ever owned
79 350 SE Euro 1996-2001- fastest car I ever owned