Foam after adding acid?

[Gibson00]

In the past two weeks, I've once per week added a cap full of acid (about 25 grams I think), Arctic spa brand, to bring the ph down. I always run the jets after adding anything. Both times when adding acid and turning on jets, I had soapy foam develop on top of the water. Only time this has ever happened. Normal??

[chem geek]

That's not normal, but it is possible for the pH of the water to affect the stability of foam -- as for what pH is more stable, that depends on the type of chemical making up the foam.

Your Arctic Spa pH Down product is likely 93.2% sodium bisulfate.  See if it lists the ingredients on the bottle.  It's unlikely they added something to the product that causes foaming.