On Sun, 06 Sep 2015 12:51:21 -0500, CRNG
wrote:

We put in a small spa last month and I'm wondering why the pH tends to
drop over time. I'm guessing that all the air that is pumped through
the water when the air jets are on somehow causes the water to
become acidic (via the CO2 present in the air). Is that a reasonable
guess?

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.