We have just recently noticed that our hot tub is producing this black almost grease like ring and everytime we lean against the back it ends up creating a black mark across our backs, it also has left a ring all around the bromaine floater we have. So, yes, we use bromaine not chlorine. I do not know what is causing this. Any help would be appreciated.
Thanks.

A few potential posibilities,

Bromine has a pretty low PH, and may turn the water very acidic. Have you inspected the spa pillows (if any) and handrail gaskets (if any) for damage? Are they in good shape, or are they slimy and decomposing?

Also, have you added anything differently into the water recently? Baqua Spa, or maybe a spa aromotherapy chemical, etc?

We don't have any spa pillows or handrails, but nothing seems to be slimy. And we haven't added any aromatheraphy or anything like that to the water ever. All of the colors on the test strip are fine except the water hardness is 0, which now I know we need to add some calcium chloride to raise it. It has always been 0 and we never knew there was a way to raise it. Until I just read some posts on here. It's really weird, I've never seen anything like this black stuff before.

Hello! I have been lurking here for a couple of weeks. A friend of mine just gave me a pretty nice (free) hot tub last week. They were having the same problem (black ring) with this tub when they had it running. I was cleaning the filter top the other day (the piece that goes inside the filter and when I started cleaning the "O" rings all of this black stuff was coming off on the towel and on my hand. You might want to replace your "O" rings if they are old. This could be your problem. There are four rings, at least on mine...one for the top itself, one for the pressure release bolt, and two for the bromine cap. The thing that spins around on the bottom of the PVC attached to the lid. You can just remove a lock ring and the two pieces will come apart. Plese forgive me if you don't have this set up. I am new to hot tubs and am learning as I go too!
Also a question for those with more experience. Are there any rubber type gaskets in and or around the pump area (or anywhere for that matter) that could cause this problem. I would like to make sure I have solved the problem before I fill this tub. Good luck solving your hot tub ring problem!
Bob

Hi Tiara33
What you are seeing may be the Neoprene from gate valves, or swing valve, or jet wear rings disolving under long periods of corrosive water conditions.
What kind of spa, and how old?