In computer water cooling loops, it's common to use a silver coil to prevent microbial growth. Could a bigger coil secured safely in the fuel tank prevent microbial growth from forming in diesel?
This is an interesting idea! You could also incorporate silver or copper into a Fuel polishing setup (simplistically just a fuel filter and pump that cycles the fuel in the tank).
You can't use UV to treat the fuel because that degrades the diesel but you could try something that floats with silver with a high surface area that would catch the algae on top or something that sinks and kills the sediment. You'd have to see if there's enough surface area on the silver to actually inhibit growth vs. just replacing your fuel filters. A good product would probably cost 3-5x a filter to make a good marketing proposition.