Well after the Physan 20 soak at 1 Tablespoon per gallon for 10 minutes I haven't seen the mold come back and I have watered using the same soak in kitchen sink method!
On another note my drain stop/plug came apart and I noticed some mold on the inner part of the rubber piece...maybe this is where the problem came from...but who knows! I don't think I ever will...lol.