Hi Gibgrl;

You added a whole BOX of borax to a 500 gallon spa . . . and STILL have a yellow pH result? Ga-WOW! You must have had straight acid in there. Any casualties?

Seriously, that is a VERY bad situation. Do this:

1. Fill a small glass with spa water. Add a spoonful of baking soda. Test. You SHOULD get a dark red/purple result.
2. If not, repeat, but use tap water. (IF you don't get purple this time . . . get another testkit!)
3. Once you verify your testkit, go park at the spa, and add borax or baking soda or washing soda until you get above yellow.
4. Then check for damage: pH that low will dissolve any exposed copper.

Wait . . . on 2nd thought. That's only 500 gallons. Change #3 to "Once you verify your testkit, check again. If you are still yellow, drain the spa!."

There's too much chance that if you raise the pH you'll stain the heck out of everything with copper that may have been dissolved. But DO IT TODAY!!!!