Muriatic Acid is a popular way to reduce pH and Total Alkalinity in a swimming pool, but does it have a place for stand alone hot tubs? Why would you choose to use Muriatic Acid in a hot tub over something like pH down (Dry Acid)?
Is it safe for a hot tub?
Let’s begin by just saying that yes, when handled with care and used responsibly Muriatic Acid is safe for a hot tub. Muriatic Acid is strong, so you want to make sure you’re wearing proper protective gear and avoiding inhaling fumes. In terms of adding it into your hot tub, you want to pour it away from surfaces and make sure that you’re pouring in a way that it mixes with the water because it can touch any spa surfaces.
You’ll want to use a calculator that is taking into account your water parameters, as well as the size of your spa, to calculate how much Muriatic Acid to add for your goals. You should never add more Muriatic Acid (or Dry Acid) than would take your pH below 7.2.
I really like the PoolMath app by the folks at Trouble Free Pool.
After adding Muriatic Acid, you need to carefully monitor pH to ensure it doesn’t go too low.
Why not pH down (Dry Acid)?
Dry Acid is effective in reducing pH and Total Alkalinity, but it comes with a build up of sulfates in the water that can damage heaters over time. If you only need to reduce pH or Total Alkalinity by a small amount, it’s probably okay to go ahead and use Dry Acid if you have it. But if you’re trying to lower Total Alkalinity by 100ppm or more, the amount of Dry Acid you need to add will be enough to cause me concerns.
You could also use Muriatic Acid to lower your initial Total Alkalinity levels and then switch to Dry Acid for small changes to pH when it rises due to aeration.