How To Balance Calcium Hardness In A Hot Tub (Lower Or Raise)



The level of calcium in a hot tub is imperative for keeping the water chemistry balanced. This level is referred to as calcium hardness.

Many people spend money to soften the water in their home, but in a hot tub, calcium needs to be present in the water to stabilize the pH and total alkalinity (TA) levels.

Ideally, you’ll need to keep between 175 and 250 parts per million (ppm) worth of calcium in the tub at all times.

Is Your Calcium Hardness Level Too High?

At times, the tub water may peak past the 250 ppm mark, which is when you’ll see issues with the water.

The reason this happens can be one of many. 

Sometimes it’s due to improperly balanced water. When the pH level is off, it also throws off the TA level, and then everything else goes haywire. 

A high water temperature can be another reason. As the temperature rises, calcium precipitates out of solution. Tubs with very high temperatures are prone to this issue if you aren’t monitoring the water regularly.

If you’re filling the tub from a water source that already has high calcium content, this could be the culprit as well. Most tubs are filled straight from the backyard tap, and this water usually has a moderate to high calcium level to begin with. 

Finally, using a high calcium chlorine such as calcium hypochlorite (mainly used for shocking), will also raise the calcium level in the water. For this reason, most spa owners won’t use this specific chlorine as a sanitizer or shock.

Why High Calcium Hardness Is Bad News

Too much calcium in the water can lead to problems with your hot tub, and will need to be rectified immediately so that it doesn’t lead to permanent damage of the tub.

  • Cloudy water. High calcium levels in the water can lead to a cloudy spa. This usually happens because there’s too much calcium for the water to hold. Think of it like adding salt to a glass of water. You mix it up, but when there’s too much salt, it becomes insoluble. Instead, it just hangs around, clouding up the water. This works exactly the same when too much calcium clouds up a tub.
  • Scale formation. You always want your tub to look it’s best, but with high calcium levels you’re going to start to see a lot of scale forming on the surface of the water, and clinging to the tub’s surfaces. It can even move into the tub’s circulation system, gunking up pipes, as well as the pump and filter.
  • Ineffective dissolving. Not only will calcium fall out of solution when there’s too much of it, but it will also cause ineffective dissolution of other chemicals in the water. This means things like your sanitizer will need to be added more, as the “proper” amount won’t be doing the job.
  • Hard to balance water. Water chemistry is a delicate balance and when one level is off, it usually throws off a bunch of other ones. Too much calcium makes it hard for you to keep your water in a neutral, balanced state. This happens because it affects the pH level, which will either spike or drop. Once the pH is in disarray, it’s open season on your water.
  • Dried out bathers. The excess levels of calcium on your skin can clog your pores. This leads to dry skin, flaking, itchy, and even breakouts. It won’t cause skin conditions such as rosacea or dermatitis, but it can make these pre-existing conditions worse. 

How To Lower Calcium Hardness In A Hot Tub

A high level of calcium in the water can only be fixed by using the following methods.

Method #1: Drain And Refill

Draining your hot tub should be done every 3 months. Unless you’re willing to pay someone to do it for you, you’re gonna have to learn it at some point.

If the calcium level is so high that it’s causing major issues with the water, you can either do a partial drain and dilute the water, or a full drain and completely replace the water.

Both are easy to do and for the amount of work that goes into it, you might as well just do a full drain.

Here’s how to drain and refill your hot tub:

  1. Turn off the tub and unplug it from the power source.
  2. You can drain the tub 2 different ways. You’ll need a long garden hose that can reach a safe place for wastewater to be dumped. This is usually a storm drain or sewer. 
  3. The first way to drain is by removing the drain plug at the base of the tub (it’s on the outside wall). Attach the garden hose to it and let the tub drain out.
  4. The other way to drain is to use a submersible pump. This pump is submerged in the tub and sends dirty water out via the attached garden hose. You’ll also need a power outlet nearby to run the pump. Ensure you turn it off when there’s no water in the tub, as this can blow out the motor.
  5. Wash down the tub using a spa cleaning product and a soft cloth so you don’t scratch the shell. Thoroughly rinse the hot tub. 
  6. If you wish, you can clean out your filter at this time too. Most hot tubs use a cartridge filter. Remove the cartridge from its housing and hose it down. If it’s really dirty, you can do a chemical soak using vinegar and water or use a filter cleaning product.
  7. Replace the drain plug and fill up the tub using the garden house attached to your outdoor water line. 
  8. Test and balance the water.

Method #2: Use Flocculant

If you don’t have time (or are just trying to avoid the amount of work involved in doing a drain), using flocculant is the other way to get rid of calcium. 

Flocculant works by coagulating the calcium particles in the water. It clumps it up and sinks it to the bottom of the tub, which you’ll then have to scoop out.

You may also come across spa clarifier, which acts similar to flocculant. The differences are that clarifier works slower, but the clumps float on the water surface for easier removal by skimmer net or by the filter.

Here’s how to add flocculant to your hot tub:

  1. Turn off the hot tub.
  2. Consult the bottle for the proper dosage of flocculant (dosages vary).
  3. Pour the proper dose of flocculant into the tub.
  4. Run the tub for an hour so the flocculant can circulate in the water.
  5. Remove any clumps that have settled to the floor of the tub. As you are removing them they may kick up. If so, let them settle and try again.
  6. Check the filter for any flocculant that may have accumulated in it during circulation. If it’s there, spray down the filter’s cartridge using your garden hose and nozzle.

Is Your Calcium Hardness Level Too Low?

The flipside to having too much calcium, is not having enough of it.

Low calcium (aka soft water), and happens when the calcium level is less than 175 ppm. Soft water has reduced mineral content (ie. calcium and magnesium), and increased sodium content, giving it a more gentle feel.

Why Low Calcium Hardness Is Bad News

As mentioned, hot tubs require hard water to stabilize the pH and TA levels, otherwise problems will arise.

  • You’ll be bathing in slimey water. The high levels of sodium in low calcium water make it feel slippery and slimy, which isn’t what you want your hot tub to feel like when you’re trying to relax.
  • It’s corrosive. Due to the lack of calcium and magnesium in soft water (as well as other minerals), it becomes corrosive and seeks out these minerals anywhere it can find them. Any metals in the tub or in the equipment line can be affected permanently as the water gradually eats through them.
  • Too much foam. Too little calcium will cause your water to produce excess foam. We all like a good foam party, but this kind of foam is pretty nasty to look at and definitely something you don’t want in your tub.
  • Damage to the tub. Low calcium water will damage any type of hot tub. Delamination of plaster shells can occur, and tile grout can erode. The floors and walls can be subject to pitting as well.
  • Bye bye, deck. If your tub is surrounded by a concrete deck, the deck itself can also be damaged via pitting. Issues like this are irreversible, so we shouldn’t have to tell you how important it is to keep calcium above 175 ppm.

How To Raise Calcium Hardness In A Hot Tub

Now that you understand why hot tubs need calcium, here’s how to add it in if your levels are low.

Method #1: Use Calcium Chloride

Calcium chloride is a highly concentrated calcium, available at 77% (hydrated), and 100% (anhydrous) strengths.

If you don’t use any other chemicals in your water that contain calcium, using calcium chloride allows you full control over the tub’s calcium level.

You may also come across a spa product called Calcium Increaser. These products use calcium chloride, but you’ll pay more for them.

Here’s how to add it to your hot tub:

  1. Turn off the tub and test the water using test strips, a liquid test kit, or digital tester. Balance the water and take note of where the calcium hardness level is currently at.
  2. Measure out the dosage your tub requires. A general rule uses 2 ounces of calcium chloride to raise 1,000 gallons of water by 10 ppm. If your tub is smaller, you’re going to have to do some math. If using Calcium Increaser, follow the dosage instructions on the bottle, as they vary between products.
  3. Fill up a bucket of water and add the calcium chloride, making a diluted solution. Mix it using a wooden stir stick so that the calcium is fully dissolved.
  4. Pour the solution into the hot tub.
  5. Brush down the walls of the tub in case there are any undissolved granules. This also speeds up mixing the solution into the water.
  6. Cycle the tub for an hour or two.
  7. Retest the water and adjust any levels if necessary.

Method #2: Use Calcium Hypochlorite

Calcium hypochlorite, better known as cal hypo, is a highly concentrated chlorine at 65% to 75%. 

As you might have guessed from it’s name, it also contains calcium. When added to a spa, cal hypo raises the calcium level in the water by 0.8 ppm for every ppm of free chlorine.

For example, if you raise your free chlorine level by 30 ppm with cal hypo, the calcium level in the water goes up by 24 ppm.

Cal hypo is mainly used as a shocking chlorine. It’s high concentration makes it ideal for shocking purposes, but the calcium from it will raise levels whenever you use it. 

This makes it not ideal in a smaller body of water like a hot tub, as calcium levels will quickly get out of control. It’s an option, yes, but one we wouldn’t recommend.

Balancing Calcium Isn’t Hard…ness

Balancing out the calcium hardness in your hot tub only requires knowledge and action. 

By learning what causes both high and low calcium levels, you’ll be prepared to tackle this issue head on if it ever becomes a problem in your tub.

Categories: Hot Tub Care, Hot Tub Chemistry