I have had the identical issue that killed many seedlings I had germed. It looked exactly like snot and I tried cleaning the res carefully and it comes back. I had a heat and poor aeration issues with my hydro setups, so I solved my heat and air issues and am waiting for GH Subculture packets to arrive and I ordered a bottle of Physan 20. How gross?! And the smell was NASTY! Thanks to all on the board for helping me get to the bottom of this issue. What a costly way to learn, but I guess that is a great lesson!!