Diesel fuel contamination in 518 fuel tank

[giant splinter]

My 518 was on rental to a fairly responsible logger for a few weeks, he called to let me know that he thought I had a fuel filter clogging up, I am not at home so I sent my mechanic up to check it out. He drained the fuel into a couple drums and cleaned the tank out, it did have some dirt, pine needles and beer can along with some crud in it so he asked the oiler on the job to pour a small sample into a can. Then tore open the filters and took everything to the dealer for some testing, the beer can was not my brand so I aint makin no appologies for that and it is to big to fit through his truck dispensing system so I don't think the logger had anything to do with that. My mechanic says he will fuel it for the duration and keep it in check for now.
After taking it to Cat and getting it checked out for algae and Bacteria (microbial contamination) and it checked out positive so he filtered the treated the fuel with some stuff and refilled the tank with it. Now I will have to deal with the problem when I get home at the end of the month.
Just wondering if any of the members have been through this and how did you treated the problem to prevent it from re-occuring?.

[Autocar]

I have locks on fuel / side panels, one time with Mountain Dew cured me.