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.

- Nathan
'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.

- Nathan