I’ve had this problem for 2 years; I’ve contacted every expert and tried every chemical I can think of…I’m at my wit’s end! I have black stuff growing on my bathtub, the tile around the tub, the built-in soap dish, and the caulk. I took a slide of it to a fungus specialist, and he couldn’t see any cells or spores, but he said that the fact that I see it growing larger and darker every day is a good indication that it’s alive. It starts to grow back as a grayish blush within a couple of days of a good scrubbing, and gets bigger and blacker every day. Nothing I’ve tried works, and no one has been able to help me. I’m not even positive at this point that it’s alive, but I don’t know what else would grow bigger every day. I hope someone can help me… Thanks!