Dennis,
When you run your pool, does your spa water continually overflow back into your pool? This is an issue as the aeration caused by the spillover will cause your pH to continuously increase.
It appears to us that your automated Intake valve (by your pump) and your automated Return valve (by your SWCG) are both mechanically set to allow water to continual flow to your Spa when you are in the Pool mode. This was done so that you Spa always has fresh chlorinated water. This is not required if you have an automation system as you can simply schedule the system to run in the "Spillover" mode a couple of times a day to keep the water fresh and keep the spillover off the rest of the time. (We can help you fix that later if you wish)
I suspect the check valve we are looking for is at the very bottom of the pic with your SWCG in it.. Off the bottom of the picture where we can't see it.
Let us know what you find..
Thanks,
Jim R.