The microbial growth (usually a fungus) requires two things to grow: water and diesel fuel.

With every heat/cool cycle of each day, if there is air space in the tank, water will condense out of the air and settle to the bottom of the tank. In other words, you will almost always have good conditions for growth.

The vast majority of the fuel we buy is sterilized when it is refined from crude oil and remains sterile until it gets to the gas station, and then if they are keeping their tanks in good order and free of water, you get sterile fuel from the pump.

The biocide will kill what is in your tank and lines. Keep dosing your tank as long as you see brown crud similar to the picture of your in-line filter above. You probably should also de-water the fuel system with Heet or something similar from a truck stop which is meant for diesel engines and removes water from the tank. If you buy 91% (or better) isopropyl alcohol from the drugstore, and dump a can of that in, it will also absorb the water, mix with the fuel, and get burned up.

Until you go through a few tanks, you'll probably need lots of filters on hand!

Alternatively, you could "polish" your fuel and clean your fuel system, if you rigged up a fuel pump and external filter (Racor is a popular brand name) and simply kept cycling the fuel from your tank through the external filter until no more dead fungus shows up in the filter. In the engine bay, attach pump intake to fuel supply line from your fuel tank, and pump/filter return to the return line to your fuel tank. You can even find filters which have a water trap in the bottom, and de-water the tank at the same time.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

ok. just wanted to make sure i was on the right track. i'll give it a few more tankfuls before i rule out a fungus/algae problem. thanks!