That's a big spa! What do you have, a swim-spa?

Regardless: add 5 boxes of borax, and have two gallons of muriatic acid on hand to compensate. Do NOT add more than one box at a time, before adjusting the pH and DO add it slowly, via the skimmer. (Borax can solidify and plug pipes!)

However, with a SWCG on a stand-alone spa, you're likely have more of a problem with this, than on most pools. The steps quoted will help you manage it, but are unlikely to make the problem go away.

BTW, thanks for the subscription.