Catfish in the fuel tank?

I've used Diesel Doctor with limited success. My 83 SD had a nasty algae problem. 2 tubes of Diesel Doctor during fill-up cleared the pre-filter and kept it clear for a few months. I never bothered to drain and flush the tank. It ran fine with or without the black stuff showing in the pre-filter.

Sixto
95 S420
87 300SDL