Red-brown cyanobacteria would make sense also.

I doubt it's root rot since I can usually wipe the black slime right off and find nice white roots underneath. I assume the slime is a biproduct of the bacteria.

10 hours after the changeout last night, pH is still 5.6-7 and the water is clear.
TestPilot Reviewed by TestPilot on . Slime and bacteria Referring to this thread: http://boards.cannabis.com/hydroponics/82722-troubling-issue-finally-resolved.html I'm getting this in my res and on the bottoms of my NFT tubes. Causes the pH to rise to 6.2+ overnight and TDS rises almost 50ppm per day. I added hygrozyme and hydroguard and one of the two managed to clear up the res in 48 hours, but the pH still crept up and took more and more ph down to adjust. Some of them are showing the effects of the pH swings. I was using GH Flora, Rating: 5