This is my second post, I'm new to TFP. I have a TF100 that I use for my pool, but I haven't yet tested my spa water because I was using dichlor and the CYA is very high. I'll do a drain and refill soon. (While I've used the TF100 CYA test for my pool, I'm basing my high CYA level in the spa on results obtained with Aquacheck test strips, which show the CYA off the high end. I don't want to 'waste' a TF100 CYA test on water I plan on replacing soon...)
Here's my question: When I drain and refill, will I need to add CYA, by whatever method, even though my spa is under a gazebo and never sees the sun, except, perhaps for an hour or so very late in the day, if the cover is off (rare)? If CYA is primarily for UV stabilization, I'd think I could skip it if the spa doesn't see direct sun.