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How to Raise or Lower Water Hardness in a Hot Tub

By Matt Giovanisci | Updated: November 14, 2019

Among all the spa maintenance tasks you perform, knowing how to lower water hardness in a hot tub is one of the most important. Equally important is knowing how to raise calcium hardness in a hot tub. Wait, which one is which? Spoiler: They’re the same thing.

Letting the calcium or hardness level in your spa get too high or too low can wreak havoc with your water chemistry, and can even damage your hot tub and its components. You need to learn how to keep it just right.

Why Is There Calcium in Your Hot Tub?

The answer is easy—because there’s water in your hot tub. Calcium is a naturally occurring element found in all types of water bodies, from creeks to rivers to oceans. It’s also in the water that comes out of our homes’ faucets because that water comes from rivers, streams, aquifers, and the ground.

Minerals like calcium and magnesium also exist in all those sources, and over time, dissolve into the water. This presence of minerals is what makes water “hard.” Depending on where you live, your water may be softer or harder than in other places. If you’re not sure how hard the water in your home is, you can get a kit to test it.

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Are Water Hardness and Calcium Hardness the Same Thing?

Another easy answer. We’re totally going to ace this one! Yes, calcium hardness and water hardness are the same thing. And now we’re going to throw another term in there: total hardness.

What is Total Hardness?

The amount of both calcium and magnesium dissolved in the water is called total hardness. Why is this important? Well, some water test strips and kits only measure total hardness, and not calcium hardness alone.

This isn’t really that big a deal. You could, if you really wanted to, do a bunch of math to determine exactly how much calcium and how much magnesium make up your hot tub water’s total hardness. But the truth is, you don’t need to. It’s enough to go by the total hardness to adjust calcium levels.

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Why is Water Hardness in a Hot Tub Important?

It seems pretty innocuous, doesn’t it? It’s just calcium. It’s found in milk and cheese and broccoli. It’s good for you! It’s not a caustic chemical like chlorine or muriatic acid. And yet, calcium can cause significant damage to your hot tub if it’s not managed properly.

Too Much Calcium in a Hot Tub

If you live in an area with hard water, you’re already familiar with scaling. You see it on your shower head, your faucets, your dishwasher, anywhere you use water in your home. If calcium levels get too high in your spa, you’ll end up with hot tub scale.

You’ll be able to see some of it on the jets, in the filter, and even some nooks and crannies in the shell. But what’s more dangerous is the scaling you don’t see.

Buildup on the pump’s impeller can cause it to seize up, and replacing the pump can be pricey. Worse yet, scaling inside your spa’s pipes can impede water flow.

Also, too-high calcium levels can wreak havoc with pH and alkalinity levels, throwing your entire hot tub chemistry out of whack.

Finally, high calcium levels can cause white flakes in your hot tub, or a cloudy hot tub. Again, you’ll have more trouble to get out of than just an incorrect calcium level.

Too Little Calcium in a Hot Tub

If you let calcium levels get too low, you run the risk of equipment corrosion. This includes the jets, the heater’s heating element, and even parts of the shell itself.

Too-soft water can also result in hot tub foam, which means you’ll have two problems to fix.

Juuust Right Calcium in a Hot Tub

We recommend a range between 175 parts per million (ppm) and 250 ppm. This number also applies to total hardness. As long as you’re in that range, you’re fine.

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How to Test for Calcium in a Hot Tub

You have three options:

Test Strips: This is the quickest and easiest hot tub water testing method. Just make sure you use strips that include total hardness.
Liquid Test Kit: A little fancier and slightly more labor-intensive than test strips, but sometimes more accurate. Again, ensure the kit tests for calcium or total hardness.
Hot Tub Supply Store: Any store that sells hot tubs and spa supplies should be able to perform a water test for you. We recommend having the water professionally tested at least twice a year.

We recommend testing for calcium—well, just testing your water in general—at least once a week. Test more often if you use the spa more often.

How to Lower Water Hardness in a Hot Tub

Did the test show a calcium level above 250 ppm? Yikes. Time to take action before you start to see any scaling.

If it’s just a little above the high end of the range, drain some of the water from your hot tub, add fresh water, and then add a scale control product to keep the calcium level where it should be. You can add scale control weekly, if necessary, but always follow the manufacturer’s instructions for use.

If the calcium level is way above 250 ppm, your best bet is to completely drain the hot tub and clean it. When you refill it, use a hose filter to keep at least some of the minerals out of the water. Then use a scale control product to help keep things balanced.

Got high alkalinity too? You can also follow our guide on lowering your hot tub’s alkalinity.

How to Raise Calcium Hardness in a Hot Tub

Did you find the water in your spa is too soft? You need to raise the calcium level before your equipment begins to corrode.

Fortunately, the easiest way to raise calcium hardness in a hot tub is to—you guessed it—add calcium to the water. Actually, you want to add calcium chloride, which is the active ingredient in calcium hardness increaser made specifically for hot tubs.

Follow the manufacturer’s instructions to get the calcium level up where it needs to be.

Dealing with Hard Water Isn’t That Hard!

By now, you should be a calcium hardness pro. Next time your neighbor asks you how to lower water hardness in a hot tub (or raise it), you’ll be able to answer with confidence. Then maybe they’ll let you borrow that sleek new riding mower they got last summer.

The main thing to remember is that a high or low level of calcium in a hot tub is fairly easy to correct. Just make sure you keep up with water testing and balancing, and you won’t have to worry about scale, foam, or cloudiness ruining your soak.

Happy Soaking!

Matt Giovanisci is the founder of Swim University® and has been in the pool and spa industry since 1995. Since then, his mission is to make pool and hot tub care easy for everyone. And each year, he continues to help more people with water chemistry, cleaning, and troubleshooting.

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