First of all welcome to the board.
Try doing a search in the diesel forum, same board but diesel forum rather than "tech help". You will find a lot of threads mentioning about fungus and how to fix the issue. I believe members used anti-algie on a somewhat regulator basis, and yes another thing to keep in mind is to get the diesel from a truck stop or from a gas station who sells big volume of diesel. The point is to get "fresh diesel". You might be getting diesel from a station which has the fungus in their tank, since they do not move the volume quick enough.
Another piece of advice, try to work on these cars yourself if you can otherwise these could be money pit. There are lot of things which can go wronb with the 15 -20 year old cars, especially if not maintained properly.

Good luck

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87 420 SEL
Looking for a nice mid 80's MB diesel.
87 BMW 528e (Sold)
99 TOYOTA 4Runner
1989 Honda Accord (Sold)
1988 Honda Accord (Total)