That blackness is algae. Get some biocide in there and start running through some tanks. Search on here there is plenty of information. Any place with diesel products will have biocide.

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1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42