Here is a patent from 1978 showing how to reduce CYA in water using sodium hypochlorite.

US Patent 4,075,094 - Sodium Hypochlorite Treatment fro Removal of Cyanurate Compounds from Aqueouss Waste Streams, Ronald H. Carlson, FMC Corp., Feb 21, 1978.

On a weight basis, this reaction requires a ratio of 259.5 parts by weight of sodium hypochlorite to 100 parts by weight of cyanuric acid. At least 90 percent of the cyanuric acid nitrogen is oxidized to nitrogen gas, the balance being oxidized mostly to nitrates. The reaction of sodium hypochlorite with cyanuric acid proceeds most rapidly at pH 9.0 to 10 and increases in rate 2-3 times for every 10.degree. increase in temperature. The effect of the initial concentration of cyanuric acid and sodium hypochlorite on the reaction rate will be discussed below. The residence time for destruction of 95 percent of the cyanuric acid present in the waste stream can range from more than 200 hours to less than 5 minutes depending upon the reaction conditions.

Interesting, but I'm not sure how applicable it is to pools. So, if you had 100 ppm CYA, you'd have to bring the FC to 259.5 (!) and your pH up to 9-10 (!) and crank up the heat. Sounds to me like a recipe for massive limescale formation!

Yeah, unless I am missing something, it appears entirely inapplicable to pool conditions.

If I owned that patent and had ten bucks, I would be worth ten bucks, I think!

This topic was already discussed in the Degradation of Cyanuric Acid thread under the sub-topic "CYA Degradation by Oxidation from Chlorine" and in a later post in that same thread. As Dave notes, it's simply not practical given the very high FC and pH levels that are required to make the reaction go reasonably quickly.

Since there are calcium hardness reducers such as HotSpring® Vanishing Act™ in a bag, I don't know why no one does something similar using melamine. Perhaps it's harder to keep melamine attached to some surface to keep it in the bag. The calcium remover may be essentially ion exchange resin or might be something that precipitates calcium such as phosphate or oxalate (though I'm not sure how they keep those from coming out of the bag).

Yes, I thought chlorine level and pH and perhaps temp were too high to make it practical. However, the residence time of 5 minutes under certain conditions intrigued me. If you could optimize the process, super-scale conditions would only exist for 5 minutes and CYA would be gone.

Melamine (the reagent used in the CYA turbidity test) was sold as a CYA reducer in the 1980s by Applied Bio Chem. It was called Melatrine. You added the powder to the pool and it created a whole pool of precipitate just like when you make the CYA reagent test. As I understand it, the precipitate was so small that most sand filters would not remove it. Then you waited for settling and vacuumed to waste and used a clarifier for the remainder. Water loss further reduced the CYA level. If the pool had a DE or cartridge filter it would work. But it required cleaning the cartridges multiple times due to the large amount of precipitate. It didn't sell much and was such a pain no one bought it.

You could use melamine in a bag but you might have the same problem with small size precipitate. I think however that 1 lb of CYA requires about 1 lb of melamine. I think the molecular weights are similar. So each 12 ppm of CYA in 10,000 gallons needs 1 lb of melamine and produces 2 lbs of precipitate.

I have heard of specific wave length UV light reducing CYA.I don't know what nm wavelength destroys CYA. And I have also heard that ultrasound might work. No other info.

this would be a good experiment for someone to try who had high cya problem. take an initial cya test, then (at the hottest time of the year) pour in many, many bottles of bleach and super duper shock the pool and then take additional cya tests every 24 hours to see if it had any effect. it might not completely eliminate it, but if it took cya down from 150 to 50 then it might be worth it as opposed to water replacement.