rust colored staining of liner steps and fittings

[jtodd8]

I have a 16 x 40 inground 22,000 gallon pool. I am having a problem with rust colored staining on all surfaces. I have a salt generator no heater and use city water. My salt cell stopped working about a month ago and I purchased a new one to the tune of $700 and this is when my problem started. Within hrs of hooking up the new cell this staining started appearing.
There was a brown residue on the discharge end of the cell took it back it was tested and I was told there was no problems with the cell. Had water tested no metals and water is in almost perfect balance. So I purchased products to get the staining out ascorbic acid (vitamin C) which cleared up the stains right away. The problem is the stains came back again the next morning. I repeated this three more times. I finally decided to turn off the cell unplug it an just use clorine tablets to prove to the pros that something was wrong with the cell. That was three days ago and seemed to me problem was solved and I had a defective cell. Nope, noticed that the rust staining was returning not as quickly or as dark nonetheless still returning. Any one have any ideas I don't what else to do please help