Waterbear, thanks for your advice, I am in England. My wife came back from Asda (Walmart in US) with vitamin C tablets with orange and zinc so when I held the tablet against the dampened stain it fizzed like crazy and turned orange which was not much help so I persisted and rubbed it around for a couple of minutes and I believe it improved the staining. I need to order some proper pure colourless 100% vitamin C tablet or powder from Amazon and try that properly and then report back. We purchased the swim spa second hand on eBay and it is an April 2018 model so not very old, when it arrived the acrylic shell was spotless, like brand new. We have only had it since May and have only ever used this Huwasan sanitizer because we saw a video on YouTube and decided to try going chlorine free. The spa has an ozone an UV system too. After reading your comments I am going to persist and try to manage and if the stains don't improve with the addition of vitamin C on recirculation, I am going to revert to chlorine or bromine. In the meantime if there is a definite improvement with the proper vitamin C tablet, how much should I add per 1000 litres (264 US gallons) of water please? How long should I recirculate for? The chelating agent I was sold is apparently the more environmentally friendly one which is L-glutamic acid N,N-diacetic acid, tetrasodium salt; GLDA-Na4. I was told to add it between 0.1 - 0.2% v/v so 1 - 2 litres per 1000 litres / 264 US Gallons. At the last fill I added fresh water then the chelating agent, waited for 1 hour then adjusted the pH (it went up from 7.3 to 8.8) then added the Huwasan silver stabilized H2O2 at 80ppm. The water goes a tinge of green for 12 - 24 hours then clears. I am finding that I have to keep adding pH minus to keep the pH in range 7.2 - 7.8. Also I didn't balance the pH for about 4 days due to going away with work and when I got back the staining had got a lot worse and the pH had shot up to 9.2!! Maybe that has something to do with the staining getting worse. Thanks for your help.