Why calcium hardness matters
Calcium hardness is one of the most misunderstood aspects of pool chemistry, yet it is critical for the long-term health of your pool’s surface and equipment. Whether you have a traditional chlorine pool, a saltwater system, or a magnesium-based mineral pool, maintaining the right calcium hardness is the only way to prevent costly damage like surface etching or unsightly scale buildup. We break down what calcium hardness means, how to identify the signs of aggressive water, and why some salt systems require a different approach to calcium testing.
| Calcium Level | Potential Impacts & Surface Issues |
|---|---|
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LOW CALCIUM |
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HIGH CALCIUM |
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How do I lower calcium hardness in my pool?
The best way to lower calcium hardness in your pool is by draining and replacing a portion of the water. This can be done through partial draining, followed by refilling with fresh water. Since tap water typically has low calcium levels—around 120 ppm in most parts of the U.S.—and rainwater or tank water contains no calcium, this process naturally reduces calcium levels without the need for chemicals.
Before starting, check if your area has hard water with high mineral content, as this could impact the effectiveness of the refill. You don’t need to drain the entire pool—in many cases, replacing just a foot of water can bring calcium hardness back into the ideal range.
If you’re unsure about your pool’s calcium hardness level, it’s best to get it tested as soon as possible. If your current pool store doesn’t offer calcium hardness testing, consider visiting another store. Since all pool surfaces interact with calcium, maintaining the correct level is essential for pool surface and equipment longevity. Balance isn’t just ranges—use the Langelier Saturation Index (LSI) to balance your water. The target is (-0.3 to +0.3 ideal) considering pH, TA, temp, and CH to predict corrosive vs. scaling water.
Top 10 Hardest Water Cities in the USA
| Rank | City & State | Typical Hardness (ppm) | Primary Source |
|---|---|---|---|
| 1 | Indianapolis, IN | 350+ ppm | Limestone Aquifers |
| 2 | Las Vegas, NV | 300+ ppm | Colorado River / Lake Mead |
| 3 | San Antonio, TX | 280 – 300 ppm | Edwards Aquifer |
| 4 | Kansas City, MO | 250+ ppm | Missouri River / Groundwater |
| 5 | Phoenix, AZ | 220 – 250 ppm | Salt & Verde Rivers |
| 6 | Riverside, CA | 210 – 250 ppm | Local Groundwater |
| 7 | Salt Lake City, UT | 150 – 250 ppm | Mountain Runoff / Wells |
| 8 | Jacksonville, FL | 200 – 240 ppm | Floridan Aquifer |
| 9 | Minneapolis, MN | 170 – 240 ppm | Mississippi River |
| 10 | Tampa, FL | 160 – 210 ppm | Floridan Aquifer / Hillsborough River |
Not sure of the calcium levels in your city water? Here is a website that will show the calcium levels in your city.
What About Calcium Hardness reducers?
Spoiler Alert! Calcium hardness reducers don’t remove any calcium. Some novice pool guys think that a ‘calcium hardness reducer’ will permanently lower the calcium level. However, this is simply not true because calcium isn’t physcially removed from the water with these treatments.
Calcium hardness ‘reducers’ work by using chemicals (usually HEDP) to bind up, or capture, calcium temporarily. They reduce the effects and test results of high calcium in the short term. However, after 4-6 weeks, the product is completely broken down and useless. Then, the calcium is released from the chemical bond and returns to normal. Many pool owners are perplexed as to why the calcium level is right back to where it started!
Some Alum-based flocculants claim to be able to clump calcium together and drop it to the bottom of the pool where it can be vacuumed out. While this sounds great in theory, many pool owners who have tried this method have seen little change in the calcium level following the treatment. Many flocculants do a great job in clearing very dirty water but often don’t do much for calcium hardness.
Topping up the pool will fix high calcium right?
Adding tap water to your pool won’t reduce calcium hardness because water evaporates – calcium doesn’t! Calcium doesn’t evaporate so topping up the pool only temporarily dilutes calcium, it won’t reduce the total calcium content.
IMPORTANT NOTE: If your source water or tap water is high in calcium, then use our scale and stain eliminator. We have many clients that have over 1,000 ppm of calcium right out of the tap! When used every 6 months, it prevents your pool surface and equipment from being damaged by high calcium.
Can I fill my pool with softened water?
Some websites warns against using softened water in a swimming pool, because a complete pool fill of zero-hardness water is highly aggressive. Without calcium, the water becomes “hungry” and will leach minerals from the pool’s surface and equipment.
However, using a water softener is perfectly safe—and highly effective—when used for partial water exchanges or “blending.” By mixing softened water with your standard tap supply, you have much more control of the final mineral content.
The ideal strategy for a gunite pool is to blend the two water sources until you reach a target Calcium Hardness of around 200-400ppm. This “sweet spot” is low enough to prevent calcium scale, yet high enough to ensure the water isn’t aggressive toward your pool’s finish.
For a fiberglass pools a calcium hardness level of 120ppm is recommended.
Calcium Hardness in fiberglass pools
Pool stores often provide poor advice for fiberglass pools because their testing software is calibrated for gunite or plaster pools. Concrete pools need 200–300 ppm of calcium, whereas most fiberglass manufacturers suggest a calcium level of around 120 ppm or less.
High calcium levels in a fiberglass pool can lead to white calcium scaling, clogged salt cells, and heater failure. To protect your pool, always follow the chemistry guidelines found on your pool manufacturer’s website rather than pool store advice.
Our Scale & Stain Eliminator prevents this white buildup from forming in your pool.
How to remove calcium buildup in pool
Spotting calcium lumps or blotches on your pool’s surface can be frustrating, but simply lowering the calcium hardness in the water won’t fix the problem. There is a common misconception that “hungry” (low-calcium) water will naturally dissolve existing surface deposits. In reality, maintaining your water at either extreme—too low or too high—can actually spread calcium lumps rather than fix them.
To treat calcium nodules, streaks, or blotches, you generally have two choices:
- An expensive drain and acid wash.
- Apply our proven calcium-dissolving compound.
We specialize in removing calcium deposits using advanced polymer treatments without draining your pool or acid washing the surface.
Suggested Calcium levels for each pool surface type
| Pool Surface Type | Suggested Calcium Hardness (ppm) | Surface Impact & Reasoning |
|---|---|---|
| Vinyl | 150 – 250 ppm | Protects metal components and prevents the liner from becoming brittle due to plasticizer loss. |
| Plaster / Gunite | 200 – 400 ppm | Prevents aggressive water from leaching calcium from the finish, which causes etching and pitting. |
| Pebble / Quartz | 200 – 400 ppm | Protects the cementitious binder to ensure quartz and pebbles remain securely embedded. |
| Fiberglass | 100 – 150 ppm | Lower levels needed to prevent white scale from forming on the non-porous gelcoat. |
| Tiles | 200 – 400 ppm | Necessary to prevent the water from dissolving minerals in the tile grout and loosening the bond. |
Calcium can ruin your pool heater in one season
While your pool water might feel warm, the pipework inside the pool heater is very hot – up to 200°F. This heat forces calcium to crystallize, coating the internal heater pipes in calcium scale. This buildup acts like insulation, making your heater less efficient, use more power and may cause it to overheat or fail. If this happens, kiss your heater warranty goodbye because pipe calcification is not covered by any manufacturer. This is why water maintenance and using scale and stain eliminator regularly is also important for your pool equipment.
Protect Your Pool from Calcium Damage
If you live in an area with hard water, or use well water, the pool’s interior finish is under constant assault from mineral saturation and scaling. In these cases, you will need extra protection for your pool. Here is how Scale & Stain Eliminator helps:
Molecular Protection: Scale & Stain Eliminator acts as a powerful sequestering agent that works at a molecular level to keep minerals suspended in the water so they cannot bond to the surface.
Dual-Action Formula: The solution is designed to both lift existing calcium buildup and prevent the formation of new calcium scale.
Surface and Equipment Longevity: Beyond protecting the interior finish (fiberglass, pebble, or plaster), the formula prevents mineral deposits from clogging and damaging pool equipment.
The “Insurance Policy”: Using Scale & Stain Eliminator is essential for extending the life of a pool in any high-calcium environment.
Calcium Hardness in Mineral Pools
Many pool owners and novice pool guys are unaware that calcium readings in magnesium salt systems differ significantly from saltwater pools. This is because magnesium-based salts also show up when testing the total hardness of your water. This leads to a false calcium hardness reading.
When using magnesium salts, around 70% of the total hardness reading is actually magnesium – not calcium. This means the tested calcium hardness will appear much higher than it actually is, unless the magnesium is taken into account.
Always factor in the magnesium content when testing for calcium. For example, if your pool shows a total hardness of 200–250 ppm, your actual calcium level is way too low and will need to be increased. If the water testing software adjusts for magnesium then you’ll need to maintain a calcium hardness of around 800 ppm, compared to the typical 200–300 ppm for standard salt pools.
Calcium Hardness vs. Magnesium hardness
Don’t let a ‘perfect’ calcium reading fool you. While 200–250 ppm is ideal for standard salt pools, that same measurement in a magnesium system indicates ‘hungry’ or calcium starved water. Without factoring in the magnesium, the water becomes aggressive and will begin to dissolve your pool’s surface to satisfy its mineral demand.
Remember: without software intervention, your water test kit reagent cannot isolate calcium and magnesium. Because of this, it simply sees them both as ‘calcium’, meaning your final result is a combined total rather than a specific calcium-only reading.
| Salt / Chlorine Pool | Magnesium Salt Pool | |
|---|---|---|
| The calcium test measures | Calcium (total) Hardness | Calcium AND Magnesium |
| Mineral composition | ~95%+ Calcium | ~70% Magnesium / ~30% Calcium |
| Target "Total Hardness" | 200 – 400 ppm | 800 – 900 ppm |
| A 200-400ppm result means | Ideal Balance | Aggressive (etching) Water |
Critical Note: If your magnesium pool tests at 250 ppm, your water is "starved" for calcium. This 800-900 ppm target ensures your LSI stays neutral and protects your pool surface.
References
Suggested calcium levels for MagnaPool
http://magnapool.com/wp-content/uploads/2016/11/How-to-maintain-your-MagnaPool.pdf
High calcium levels damage fiberglass
https://www.swimuniversity.com/calcium-hardness-hot-tub/
Average tap water hardness in the USA
https://homewater101.com/articles/hard-water-across-us#:~:text=An%20overwhelming%2085%20percent%20of,of%20hardness%20across%20the%20USA.
Suggested Calcium hardness in Fiberglass Pools
https://thursdaypools.com/fiberglass-pool-water-chemistry/#:~:text=Excessive%20amounts%20can%20diminish%20chlorine’s,avoiding%20equipment%20corrosion%20or%20scaling.
Pentair Heater Warranty – Pipe calcification
https://www.pentair.com/en-us/education-support/residential/product-support/warranty-information.html
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