It does seem strange that your CYA is not dropping very much with each water change. Are you confident in your CYA test results?
Here are the detailed CYA testing instructions:
CYA - Cyanuric Acid
Fill the mixing bottle to the lower mark with pool water. The lower mark may be labeled as 7 ml, 15 ml, or not labeled.
If the pool water is below 70 degrees, allow the water sample to warm up to room temperature before continuing.
Continue filling to the upper mark with R-0013. The upper mark may be labeled as 14 ml, or not labeled, or there may not be a mark and you fill to the base of the neck of the bottle.
Cap and then shake the mixing bottle for 30 seconds.
Stand outdoors with your back to the sun and hold the view tube at about waist level. If sunlight is not available, find the brightest artificial light you can.
Looking down into the view tube, slowly pour the mixture from the mixing bottle into the view tube.
Continue pouring until all traces of the black dot at the bottom of the view tube completely disappear, even after you stare at it for several seconds, or you fill the view tube.
Reading the result:
If the view tube is completely full, and you can still see the black dot clearly, your CYA level is zero.
If the view tube is completely full and the black dot is only partially obscured, your CYA level is above zero but lower than the lowest level your test kit can measure (20 or 30 ppm).
If the tube is not completely full, look at the scale on the side of the view tube. The labeled mark closest to the liquid level indicates your CYA level.
If your CYA level is 90 or higher, repeat the test adjusting the procedure as follows:
Fill the mixing bottle to the lower mark with pool water.
Continue filling the mixing bottle to the upper mark with tap water.
Shake briefly to mix.
Pour off half of the contents of the mixing bottle, so it is again filled to the lower mark.
Continue the test normally from step 3, but multiply the final result by two.
Notes
If you are not confident in your reading, you can pour the solution from the view tube back into the mixing bottle and then try filling the view tube again. You can repeat this portion of the test over and over again for several minutes without affecting the test results.
This page at Taylor Technologies has photographs which show what the view tube looks like when the test is complete.
Very rarely, cloudy or murky water can affect the CYA test. To check for this, fill the view tube with straight pool water. If you can still see the black dot clearly, the cloudy/murky water is not affecting the test result.
View tubes and mixing bottles come in two common sizes. One set requires 7 ml each of pool water and reagent, and reads CYA levels down to 30. The other set uses 15 ml each of pool water and reagent, and reads CYA levels down to 20.
It is also possible to get a tube with a sliding black dot, Taylor 4088, so that you move the black dot up and down in the sample, instead of pouring the sample into the view tube.
If you are using the large view tube and know that your CYA level is at least 50, you can use half as much pool water and half as much R-0013 to save on reagent.
R-0013 contains melamine, which binds to cyanuric acid, forming the white precipitate which turns the sample cloudy.
The precision of the test, when done correctly, is around plus or minus 15 for levels up to 90 and plus or minus 30 for levels between 100 and 200. Novices often have problems reading the test correctly, and tend to get higher than actual readings.
