Treat your tank for the water, this is the food source for the micro growth. Adding biocide only kills the micros and then the filter is the receiver of the unwanted dead. As a professional in the tank cleaning, I give my boating customer three very strong recommendations:
1) keep your tank full or empty
2) treat your fuel by the size of the tank (if you have a twenty gallon tank and only ten gallon of fuel, treat it for the twenty) it's the dead air space in the tank that generates the condensation
3) get a six-pack & go boating (a six-pack of fuel filters)

I would also recommend the same for these older diesels.

Good Luck