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Chlorine levels jump AFTER shocking with MPS

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I've owned (and successfully managed water in) a hot tub for a decade and am on tub #2 and have never encountered this before. When my total chlorine is running high but my free chlorine is low, I shock my tub with MPS (LeisureTime Renew as recommended by dealer for a Sundance Edison... want my Hartford back!!!) After 20 minutes of circulating the chlorine readings jump - total chlorine is off the charts and free chlorine jumps into the "high" range. What??? I used to use MPS to LOWER chlorine levels if needed, and thought that it was just as (or more) effective in burning off the byproducts as the free chlorine. I'm completely puzzled by this and wondered if anyone has an explanation? Some details: water is old due to water restrictions in our area, but everything except the chlorine is in balance and easy to keep there. However stabilizer is on the low end considering I've been using dichlor ever since it dropped, somewhat inexplicably, a few months ago. This tub has an ozonator and a "clear-ray" bulb (?) both gadgets I didn't have in my last much-easier-to-maintain tub. Clear-ray bulb is old enough it should probably be replaced. I don't know of any maintenance requirements for ozonator. This tub also has 1 filter rather than 2 (I actually miss the good old days of cleaning 2 filters but having sparkling water to show for it!) Any thoughts or advice welcome!

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Ok. Where to begin?

Mps in the water will give a false sanitizer reading. As it remains present in the water until it has oxidized a contaminant, having few or no contaminants to oxidize will give you false readings. It will also throw off your ph, by the way. Possibly cya too.

Ozone is an oxidizer, much stronger than mps, and it burns off the chloramines (combined chlorine) and other organic contaminants including free chlorine in the water. UV has a similar effect on chlorine and chloramines. I use ozone and do not "shock" unless I spill a drink in the spa. The ozone does all the shocking I need for normal use. I also do not try to maintain a residual, as the ozone will make that very difficult. Doubly so with UV. I add chlorine after each use to destroy contaminants and let the ozone do the shocking, so next time I open the cover I have clean, odor-free, virtually chlorine free water to use. I also use nature2 to keep it clean when not in use and deal with any contamination that may occur with the cover on.

Ozonators do burn out over time. You should check yours.

Stabilizer does not drop. You can't realistically get rid of it without draining and adding in fresh water. Your cya did not drop, something else is going on. How are you testing?

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