Hey guys, new member here. I just purchased a house that has a hot tub. I believe it's been sitting 6-12 months. I cleaned with Oh Yuk thinking it could probably use a couple of cleanings, but here I am after over a DOZEN cleanings, and I'm still getting basically the same amount of black stuff out of the jets as the first time. I'm guessing it's mold - a black dotted substance. But there's alot of it built up on the sizes of the tub after each cleaning - probably a 1/2" wide line of this black stuff around the entire tub. I'm following the instructions exactly, plus some - fill with water to the top of the jets, pour in 4oz Oh Yuk (I use 8oz every couple cleanings just to boost it), let run for a while (hour to an hour and a half each cleaning), drain, wipe residue, fill with water, run jets to rinse. But every time I run jets to rinse, there's still black stuff in the water. I'm not sure what else to do, is it time to call a professional to disassemble the entire mechanism? Or do I just keep cleaning until it's gone?