I have a customer that has a Ace spa that is having major issues. At this point I have exhausted all ideas of what might be going on here. Customer has well water and the water has been tested. The only things present were minuscule amounts of magnesium and iron is present. The other thing present is Radon for which they have treatment system(External hose is not going through softener system). Customer filled spa with Watkins pre-filter on a slow trickle as required. We also used the Vanishing Act to remove hardness and get in appropriate range. We currently have used 5 bottles of metal gon to remove all trace minerals. Metal gon was applied daily for 5 days. The filter was removed and rinsed daily to ensure that any excess minerals were removed. The issue we are having is that the water will be crystal clear for a week or so and then will slowly start to turn a yellowish brownish hue out of nowhere. I told the customer not to add any un-treated water to ensure there were no minerals being put into the spa. The Ace cell has been tested numerous times and is fully operational. The only factor that I can not rule out is the Radon. I do not have any experience dealing with water that has Radon present and or the effects of it on hot tub water chemistry. I will be sending out a water sample to have analyzed but it takes roughly 2 weeks. Any help would be greatly appreciated I am at a loss with this one.