spa pH??

Do you know the normal ph of the water you add to the spa, or the ph of the chemicals that you add to the water?

The short answer is the stabilizer in your sanitizer is acidic, along with rain water. If it is a plastic tub, the pH will drift lower. I exchange mine with the concrete pool and that stabilizes it, actually jacking the number up a little over time so I have to put a splash of acid in the pool. You will be using soda. A little dab will do you. Start around a quarter tsp and check it tomorrow. It is better to sneak up on the right number than to overshoot.

But shouldn't the hydrogen hydroxide make it turn basic?

I have a covered one and it's PH tends to gradually drift up over time too. Like you say, not really much of a problem, I just add a dash of sodium carbonate if it needs it. I would guess it's caused by the oxidizer too, most of them have some effect on PH. With pools for example, if you use bleach, it raises the PH.

CRNG presented the following explanation :

Hire a pool man to take care of your spa. That way all you have to do is heat the water, turn on the water jets, get in and relax. That IS why you installed a spa, yes?

Thanks to all for the comments.