Are you typing about a stain on the inside of the tub where the hot water returns?
If so, it is most likely just a reaction to iron in the water - ozone tends to make it worse if you have a system.
8-)
Yes, I'd take a picture if I could but I doubt it would come out. The heater at the bottom, where the HOT water comes out if you turn the heat up... Bubbles come from it, and hot water.
There is a brown rust like color down there, leaving a mark on the bottom of the tub.
I can clean it when I am inside and I wipe the bottom of the tub with a brush, but I am wondering if it is the water or a part gone bad...
The water is 5 months old.