Hard Pool Water: Removing Calcium Scale


Hello, I’m Dr. Pool, resident chemist at In The Swim.

Today’s discussion centers on calcium scale in the form of calcium carbonate, also known as lime scale.

Many parts of the US have hard water which can leave calcium deposits on pool surfaces.

What causes Scale in Pools?

Calcium carbonate scale can form along the waterline of the pool as evaporation increases during the summer.

Calcium crystals can form as nodules dotting the surface of plaster and metal surfaces or as crusty deposits in corners.

The cause of calcium scale is the result of high calcium hardness, over 400 ppm, and is triggered by high Alkalinity, high pH and high water temperatures.

How to Prevent Calcium Scale?

  • Use a Sequestering Agent to keep calcium locked up in solution, so it can’t precipitate into scaly deposits. Agents such as Scale Free, Stain Away and Metal Klear sequester metals and minerals.
  • Avoid the use of calcium hypochlorite shock, which will add calcium to the pool.
  • Keep your pH and Alkalinity on the low end of the scale; 7.2-7.4 pH and 80 ppm Alkalinity.
  • Balance your LSI. A Langelier Saturation Index determines your pool’s propensity to scale. Pentair LSI calculator accepts 5 water test results and then does the math for you.
  • Use a clarifier if needed to seek out calcium particles clumping them together so that your pool filter can remove.

How to Remove Calcium Scale?

Calcium Scale can be removed with muriatic acid, or dry acid dissolved in water. A sponge works well for small areas or a flower watering can for larger areas. Don your lab glasses and gloves for acid removal of tile scale. It will fizz and bubble and emit noxious fumes as it dissolves the scale. Acid Magic can also be used.

Tile with calcium hard water deposits, before and after. Photo by cleanthatpooltile.com

Calcium Deposits on Tile:

Lower the water level to the bottom of the tile and prepare an acid solution and apply with a sponge or thick brush. A putty knife or wire brush can help to knock off scale deposits. Rinse with water after 1 minute, reapply if needed.

Use a Pumice Stone or PoolStone to remove scale deposits from pool tile with a lot of elbow grease.

Hire a bead blasting service to come out and professionally remove the calcium on pool tiles. Many are now using magnesium sulfate, an inorganic salt that is less aggressive than bead or sand blasting.


Calcium Deposits on Plaster:

Many white plaster pools have calcium deposits all over them, but you don’t notice it because the surface is already white. Black or Grey plaster pools develop the same hazy calcium films but are quite noticeable. Darker colors can be restored with acid washing.

To remove nodules or crystals from the surface, you can start with a PoolStone, and see if you can manage to do the entire pool, sanding off the rough calcium bits. Or, you can drain the pool and acid wash, or possibly just pressure wash the pool.


Efflorescence on Tile and Stonework:

Similar to calcium scale which comes from the pool water, efflorescence is calcium lime scale that comes from the grout between tiles or stones placed into a wall or water feature.

What causes this is moisture coming out from behind the tile or stone through the grout or mortar joints and bringing calcium with it. The same process that creates stalagmites.

Sometimes the moisture comes from the pool as shown below. Sealers can be used on the tile to provide an invisible barrier. For larger stone or brick walls above the water, preventing efflorescence may require sealing for sources of water behind the wall.


For those of you that live with hard water (>400 ppm) right out of the tap, you have my sympathies!

But even with hard water, you can control the calcium and prevent deposits with good water balance, sequestering agents and a little elbow grease!


Dr. Pool



Hard Pool Water: Removing Calcium Scale — 19 Comments

  1. Hi. I have a new pool and spa with glass tiles around the inner edge. Already, there is lots of calcium that looks to be dripping down from the bottom of the coping. My pool guy said to use a pumice block, but that seems way too abrasive for the glass tiles. What do you recommend?

    • Hi, calcium from the coping can be acid washed off, after scraping off the big stuff with a screwdriver, putty knife, wire brush or yes, pumice stone. Test a small area first, but most glass tiles are more scratch resistant than you may think. To slow the efflorescence, consider sealing the coping stone and mortar bed above the wall, with a clear concrete sealer. Reapply every year or two. You can also seal the glass tile, to reduce the bonding of the calcium, making it harder for it to stick.

  2. we have a tile pool with a very heavy calciumdeposit on it. We have tried to clean with muriatic Acid but doesn’t seem to work. Could with use a sander with a finer sand paper so off it then muriatic once again?

    • Hi, for rough efflorescence I usually scrape it off first with a small putty knife or large flathead screwdriver. Sanding may work, but it is usually kind of wet, so it may gum up quick. You can scrape pretty hard on most pool tile without damage to it, so you can get most 90% or so, off manually, and then hit it with acid to finish, maybe twice. A plastic or soft steel rotary wheel on a (cordless) power drill would also work fairly well.

  3. If a pool is in a hard water area and using Calcium hypochlorite, ideally it should change to sodium hypo but in the interim I have heard adding salt tablets to the skimmers can reduce scaling and calcium build up. Is this true

    • Hi, I’ve not heard that before, and my thought is that it would not be true. Using a sequestering agent will help control scale, but calcium will still rise. You can also use Di-Chlor pool shock, or non-chlorine pool shock, to stop adding additional calcium to the pool, from Cal Hypo.

  4. i have bad hard water calcium on the tile above the water line. Tried muriatic acid and it didn’t do much. Tried pumice stone, and its too much work. Will renting a pressure washer with 2000+psi work to remove it? its pretty thick calcium. Rather not pay $$$ for professinal to come bead or soda wash….

    • hI Monty, yes you could use a pressure washer, however, if the tile or grout is loose, it could cause some damage, so be careful. Standing in the pool will be more precise than standing on deck, I suppose. Putty knives or screwdrivers are also good to pop off calcium scale. Acid wont do much for thick deposits, but is good for thin films. Muriatic acid in a spray bottle is my method. Often takes 2-3 treatments. pH will drop of course, use 1-2 lbs of ph increaser for each gallon of acid used, to raise pH again. So yes, the pressure washer will work, but go easy, just get the big stuff, then finish up with some acid to remove the filmy scale left behind, as needed.

  5. I have a above ground 15,000 gal. vinyl liner. This year I starting using aqua silk chemicals. (Last year I used softswim from the pool store)
    Test strip readings
    PH : 7.2
    Alk: between 40-80
    Biguanide: between 30-50
    Total Hardness: 0

    My question is what level should the total hardness be? The water stays slightly cloudy. Can I use any brand of calcium raiser with aqua silk?

    Thank you for the help

    • Hi, it might be unlikely that your total hardness level is zero. The recommended level is 200-400 ppm, and yes you can use any brand of calcium hardness increaser with biguanides.

  6. I have stainless pool walls. It looks like I have scale build up below the surface line. I could not see it when it was full. I emptied the pool to paint and it extends to the bottom of the panels. Any idea how to remove without acid?

  7. What about a vinyl pool. I have learned about calcium scaling way too late and now have a swimming pool with tons and tons of calcium deposits all over the liner particularly in places where there are divots? What are your step by step recommendations?

    • Hi Rose, great question, we seem to have forgotten to discuss vinyl. Sounds like you may have crystalline deposits? First thing is to balance the pH and alkalinity. Test and adjust pH to 7.2 or so, and alkalinity between 80-100 ppm. keep a good chlorine level and start running the pump and filter a bit longer than usual. Add a good sequestering agent, (a stain & scale) like Stain Away, or Scale Free, to lock up the minerals, back into solution. Brush the pool daily, buy a new brush if yours is old and small, to help loosen the nodules. Check pH daily, and make sure it stays low in the 7.1-7.3 range. If no improvement after 3-4 days, you can carefully take the pH a little lower, into the more acidic 6.8+ range, but only for a day or two, while brushing, to help dissolve. Then raise pH again to 7.2-7.4, and adjust alkalinity again if needed, to 80-100 ppm.

  8. Does efflorescence occur on tiles that are below the water line i.e. does it require some interaction with air to occur?

    Aesthetically, we’d prefer to line our pool with stone pool tiles (rather than glass mosaics), but are concerned that even if we pre seal them, they will be affected by efflorescence.

    • Hi, efflorescence occurs from the rocks leaching the minerals from the backside of the pool, into the pool, and moisture on the front side accelerates it. This does occur when stone mortar is below water level, but it just dissolves into the pool, so you never see the stalactite-like remains, as you do above water level.

      The best way to prevent efflorescence is to seal not just the front side, but the back side. Prevent ground water and pool water from getting behind the wall. if there is a deck over top of the raised wall, that will stop much of the water, but not all. Sealants or membranes (rubber layers) can be used on the outside of the wall to keep the wall dry, along with effective de-watering design, (gravel and pipes and underground swales), to channel water away from the area.

  9. Our pool was built in 1972. It has fiberglass walls and concrete floor. Can I use something like car (Turtle) wax on the walls to remove the oxidation and restore the color?

    • Hi Jacen

      Yes, you could use any fiberglass auto/boat restore product, or rubbing compound, and a coat of wax sure. I don’t think you’ll get too much color back, but it wouldn’t hurt. Test a small area and see how it does! You could also sand it and paint with epoxy pool paint ($$), or you could even tile the top foot of the walls ($$$), which looks really sharp. Or, there may be some mobile gelcoat or fiberglass resurfacing companies in your area that can apply a new gelcoat ($$$$). But these are just fanciful ideas. There may be a sort of product that can ‘stain’ the surface, but I’m not aware of one.

  10. I had no idea that there are chemicals that are useful for calcium buildup removal. I also had no idea that you could use pressurized sprayers to more effectively remove calcium buildup. I’ve heard it’s a good idea to do pool cleaning and maintenance weekly, I’ll have to look more into it though.

Leave a Reply

Your email address will not be published. Required fields are marked *