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A couple thoughts. One, could the problem be with the regents in the test kit? You mentioned it was old. Two, do you have a small, local pool and spa store that would test your water sample for free or practically free? Three, the water company is not going to notice 480 gallons on your water bill. Just don't let the neighbors see. A nice soaking bathtub can easily hold 90 gallons of water. What if you have several kids to bathe? Maybe you did some extra loads of wash, wink wink. What would I do, first take a sample of water to be tested. 2nd, if needed, refill the tub. Good luck.
You go a full 6 months before a water change? And you usually spend the 6 months not testing the water for pH, alkalinity, etc., levels? You just add dichlor after each use and shock every 3 weeks? No MPS or other chemicals? Do you check your FC, TC, and CC every week? Month? They say that the water needs changing when you have trouble getting the water balanced. Seems that is the case and certainly at the 6 month point, I don't think adding half new water will change things. I am new to this, but I am spewing forth all I have learned.
Based on what you have said, you could have a high cyanuric acid (CYA) level. This chlorine stabilizer is in in dichlor. If you use dichlor regularly and shock with dichlor, the CYA will build up. Nothing can reduce the level except replacing the water. A high CYA level can through off chlorine readings. I know a lot of folks on this forum recommend not micromanaging the water chemistry and they are right. But you do have to keep up with it with a reliable testing method (preferrably Taylor 2005) on a regular basis. Dump and fill (at 3 AM if necessary).
I just thought I'd give everybody an update on my efforts. The water is back in balance. No tub drain or refill. Just careful adjustments (baby steps) on the balance issues. One thing I CAN say for certain it that this recent effort has added FAR, FAR more chemicals to my water than I have EVER put in there before so if TDS levels weren't high before they almost HAVE to be now. Now that it is back in balance (and staying there) I am going to take a sample to the local Spa shop (they sell DimOnes) and see what the TDS's are as I have no way to do that myself. If they say they are high then I will dump the water even though it is now under control. BTW - What is a high vs. acceptable level of TDS in a 480 gallon tub? Never could measure it so I never captured that aspect of water balance. I think it's ppm so I guess tub size doesn't matter, only the ppm does, right?