Pool / Spa Issues - Skin rashes, mustard algae

[SunnyOptimism]

A few other points to make -

Testing -

During this cleanup, testing can become tedious. The easiest way I found to test during this shock procedure is to only test FC/CC and to use the 10mL sample size which translates into 0.5ppm/drop of chlorine titration reagent. Also, if you keep a good log of FC and CC, you should be able to see the trend easily and then you can dose regularly during the daytime without having to test each and every time. The critical times for testing are in the morning and evenings (1st test of the day and last test of the day) as those are the ones you use for measuring your overnight chlorine loss.

Reagents -

Shocking a pool can very quickly use up the DPD powder and chlorine titration reagents found in a K-2006 test kit. If you can, order more refills BEFORE your start. Another way to save on reagents would be to dilute your water sample in half with DISTILLED water and then multiply your results by 2. I would only do this for the less critical testing times during the day and not the tests for the OCL. When you dilute your sample, the +/-0.5ppm accuracy of the 10mL sample now becomes more like +/-1.0 ppm. So your tests is less accurate and not appropriate for OCL determination.

[SunnyOptimism]

I was reading you post a bit more closely and I see you have a tablet feeder. How long have you been using it? Do you do regular water draining and refill?

I ask because trichlor tablets add a lot of CYA (stabilizer) to your water over time and, without regular water exchanges, you can easily build up CYA in your water over 100ppm. If I were you, I would retest your CYA again but do dilution testing. Take a sample of pool water and dilute it 1:1 with DISTILLED water. Test your CYA and then multiply the results by 2. If you need further dilution, you can take 1 part pool water and add it to 3 parts distilled water, test CYA and multiply by 4.

The CYA test is tricky and it can be interfered with by high FC or pH out of normal ranges. It's also best to allow the water sample to come up to room temperature if the pool water is cold as the CYA test should be performed on water that is at least 68F-70F.

During a shocking procedure to remove algae, you need to get all of the tricolor tablets out of the feeder. The extra CYA is not helping you. Long term, I think you need to ditch the trichlor tablets and just use bleach in your pool as the stabilized chlorine products just cause more trouble then they are worth. But that's a conversation for another day.