At the very least, replace the fuel hoses on both sides of the clear plastic fuel filter. If they're the old braided-cloth type hoses, they're probably the culprit. Replace them with rubber hose. The dealer game me about two feet of rubber fuel hose for free when I bought both fuel filters. I guess I paid for it, though, through the usual dealership markup.
Anyway, I had the exact same problem a few months ago, and replacing these hoses cured it instantly.
'83 240D, 250k miles
P.S. Maybe the algae you killed with the biostat was what was stopping the leak. When the algae went away, the leak opened up.