I take my cuttings in put them into a DWC cloner where the water comes up to the bottom of the cutting. I circulate the water with a regular air stone that goes into the 1.5 gallon DWC cloner. I get a brown gunk on the bottom of the stems thats in the water and I have to wipe them off every few days. I think the junk might be coming from the air stone, but I change them out and the gunk is still there. Is there a plastic air stone out there that will circulate the water without getting a build up of shit in it? Also is there anything I can add to the water to help fend off this gunk and stimulate the rooting cycle?