How to Lower Calcium Hardness in a Pool (7 Steps)



Is your calcium hardness level too high?

Excessive calcium can lead to scaling, cloudy water, clogged filters and plumbing lines, and even a compromised saltwater generator.

We’re going to walk you through how to properly lower the calcium hardness level in your pool, including the pros and cons of each method.

When is Calcium Hardness Too High?

For plaster or fiberglass pools, a calcium hardness level of over 500 parts per million (ppm) is generally less than ideal and will often benefit from a reduction.

For vinyl pools, you have a bit more wiggle room of up to 600 parts per million.

But it’s still not quite that simple.

The amount of calcium your water can hold depends on the saturation of the water, often referred to as the CSI (Calcium Saturation Index). You can calculate this number using our CSI calculator.

Put simply, if the CSI of your water is negative, your water can hold more calcium which means you can maintain a higher level of calcium hardness before it becomes a problem.

This is why hard limits can be misleading, and why it’s often best to wait for signs of scaling before jumping to conclusions.

Overall, if you’re not seeing scale formation (or a thin, white film) on the surface of your water, you probably don’t need to lower your calcium hardness.

Can You Use a Chemical to Lower Calcium Hardness?

No, there aren’t any chemicals that lower your calcium hardness level.

While there are calcium hardness reducers, these are just a special kind of sequestrant that temporarily binds with calcium to prevent scale formation.

The calcium is still in the water, and it still shows up on a calcium hardness test.

After a few weeks or so the sequestrant will begin to wear off, and the calcium soon deposits itself along your surfaces as calcium scale. You could continue to add more reducer but this isn’t a cost-effective solution in the long term.

In short, the only way to remove calcium from your water is by throwing the baby out with the bathwater, so to speak.

How to Lower Calcium Hardness (7 Steps)

Unlike some substances in your water, calcium doesn’t degrade over time.

For that reason, the most practical solution for lowering calcium hardness is to drain some of your pool water and replace it with fresh water.

Here’s what you’ll need for a partial drain:

  • A reliable testing kit
  • Waterproof sticky tape (optional)
  • A tape measure (optional)
  • A submersible pump with an attachable hose
  • General pool chemicals for rebalancing

1. Test Your Pool and Source Water

You need to know your pool’s current calcium hardness level before you can figure out how much water you need to replace.

You will also need to test your fill/source water to make sure it doesn’t contain a high level of calcium, otherwise, you could end up right back where you started. This reading will also be required in a later step.

You can use test strips for this part but a liquid kit using reagents will always give more accurate results.

2. Decide on Your Target Level

If you have a plaster or fiberglass pool, you generally want to stay between 300 and 500 parts per million, with 400 ppm being ideal.

However, if your calcium hardness tends to rise over time, consider targeting 300 ppm to buffer against the inevitable increase.

For vinyl pools, calcium hardness isn’t much of a concern because vinyl liners are immune to leaching and etching caused by low levels of calcium. Basically, you can target a number as close to zero as feasibly possible.

The only thing to keep in mind is your pool equipment. Some companies even require that you maintain a minimum calcium hardness level as part of the product warranty.

3. Run the Numbers Using our Calculator

We built a calcium hardness reduction calculator to make this easy.

All you need to do is plug your test results into the fields below along with your target calcium level, and it will tell you exactly how much water to replace.

If you don’t know your pool volume, use this calculator first.

4. Mark Your Water and Drain Level

If you don’t have a flow meter to track the amount of water leaving or entering your pool, you can use some waterproof sticky tape instead.

Before you start draining, mark your current water level with a strip of tape. This will ensure you replace exactly the same amount of water you took out, which will help everything play out closer to your calculations.

It’s also helpful to mark the “drain to” point below the surface of the water based on the percentage of water you need to drain, so you know when to stop draining the pool.

For example, if you need to drain 30% of the water, you can measure the distance from the surface of the water to the bottom of the pool, and multiply that by 0.3. The resulting number is how deep you place a strip of tape.

5. Partially Drain the Pool

Now that you know how much water to drain, it’s time to start spilling.

If you’re only making a slight adjustment (draining a small amount of water), backwashing through your filter will suffice.

For any meaningful reduction, a submersible pump is the way to go.

Simply turn off your main pool pump, attach a hose to the submersible pump, and carefully lower it in the deep end of your swimming pool. Be sure to have the end of the hose in position before turning the pump on.

If you have a flow meter attachment for your hose, you can stop draining as soon as you hit the number of gallons in step 3. Otherwise, the sticky tape marker is your best friend.

6. Refill the Pool with Fresh Water

Attach a hose to your spigot, and place the end of the hose in your pool before turning it on.

Depending on how much you needed to drain, this could take anywhere from a few hours to a few days, so you’ll need to monitor it closely throughout this period.

As before, if you have a flow meter, you can simply turn off your water source once you’ve reached the number of gallons required. If you don’t have a meter, you’ll need to rely on the sticky tape you placed at the water level in step 4.

Finally, once filled back up, don’t forget to turn your pump on and allow enough time for a full cycle to mix the new water in with the old.

Note: If you removed a large volume of water or you’re in a hurry, you may want to call a water delivery service as this will be a lot quicker.

7. Test and Rebalance Everything

Draining and refilling dilutes everything in your water, not just the calcium.

At this point, you’ll need to do a comprehensive test of your water to see where you stand, including your pH level, total alkalinity, calcium hardness, cyanuric acid, and free chlorine.

While you can expect your calcium hardness to be well adjusted, you’ll have some work to do in rebalancing the rest of your water chemistry — particularly your free chlorine and cyanuric acid levels.

And yes, you may need to do more than one round of testing and readjusting before the pool is safe to use.

How Do You Prevent High Calcium Hardness?

Aside from your fill water, the only way calcium is introduced to your pool is if someone introduces it.

Calcium chloride is often used to intentionally raise calcium hardness, but calcium can also be found in some chlorine shock treatments to the surprise of many pool owners.

Specifically, if you’re using calcium hypochlorite (or cal-hypo) shock, you’re adding more calcium to your water with every bag. Depending on the amount of shock treatment used, this could raise your calcium hardness by as much as 20 ppm overnight.

In other words, as long as you remain mindful of what you’re putting in your pool, your calcium hardness will stay relatively still.

The Bottom Line

A high calcium hardness level is a notorious problem to deal with because it’s one of the few pool-related issues that can’t be patched up with more chemicals.

Fortunately, by taking a moment to understand what you’re putting into your water, it’s not something you’ll have to deal with again anytime soon.

Categories: Pool Care, Pool Chemistry