From the Department of Energy Backup Power Work Group Best Practices Handbook for Maintenance and Operation of Engine Generators, Volume 1, page 22 of the linked PDF.
http://www.osti.gov/bridge/product.biblio.jsp?osti_id=491576
"It is important to realize that biocides and stabilizers are not effective in correcting existing fuel contamination problems. Extreme caution should be taken not to treat a tank that is in service with a known problem. Stabilizers will not reduce the combustible particles in fuel, but will prevent more particulate from forming in the fuel.
Furthermore, if a diesel fuel tank has large microbiological colonies treating the fuel tank with biocide while in service may result in engine failure if the colonies are killed and float to the engine suction line. If a tank is known to have severe to moderate biological growth problems the tank should be taken out of service and cleaned prior to adding a biocide."
In short, you killed the growies and they are now clogging your in tank screen.
-Jim