How to lower calcium hardness in your pool

All water contains a certain amount of calcium carbonate. The mineral concentration of your water depends on where the water comes from and therefore where you live. Limescale occurs when the mineral content of your water is high (meaning the water is hard). These minerals generally come from the ground rocks that the water travels over in order to reach your home. The scale left behind by evaporated hard water is primarily made up of calcium carbonate (CaCO3) and magnesium carbonate (MgCO3).

It's a good idea to check the level of water hardness where you live before filling your pool. If you are are using tap water, simply check the hardness of the water from an indoor water point.

How to measure water hardness

The mineral content of your pool water is generally referred to as Calcium Hardness or CH, and is measured in parts per million with the ideal level sitting around 200 to 400 ppm. Total Hardness (TH) refers to the amounts of both calcium carbonate and magnesium in the water. The higher the TH, the harder the water. The lower the TH or CH, the softer the water.

• 1 ppm = approximately 1 mg/L 

A pool water softener, or sequestering agent, is designed to combat the harmful effects of excessively hard water. If left to build up in your pool, limescale deposits will eventually damage your pool liner or prevent your equipment from working as expected. 

To avoid these types of problems, you simply need to pour the correct product directly into your pool or place a solid tablet into the skimmer. The quantity you need to use and how often you need to treat the water depends on the size of your pool and the hardness of the water. 

The south east of England generally has hard water while Scotland, Ireland and Wales have soft water. 

Ideally, the calcium concentrations in your pool should be around 250 to 350 ppm. 

How to treat soft water

Water that has a ppm of just 10-50 is generally considered too soft. In this case, the water is too acidic which means it can have a corrosive effect on your pool equipment.

The effects of hard water on pools 

If your water is too hard, scale will build up on your pool liner and may clog up filters and pipes. The warmer the pool, the more this tends to happen.

No matter whether your water is too hard or too soft, there is a treatment to help. If you fail to treat your pool water correctly, you could end up damaging your liner or equipment. 

A sequestrant is generally used for hard water. If your water is too soft, calcium carbonate is usually the answer.

Hard water should be treated to avoid harmful effects both in the short and long term:

• Water with high calcium hardness (i.e. hard water) is irritating for the skin and eyes.

• If left to build up, scale will form deposits that can make the liner of your pool rough to the touch. This also gives algae a surface to stick to.

• White stains will appear and your pool liner will get damaged much more quickly.

• Your filters will also see scale build-up which can lead to blockages and make your filtration system less efficient.

• The pool water may become cloudy.

Calcium Hardness can be tested using a swimming pool test kit.

Water hardness test strips 

The easiest way to measure the Total Hardness or Calcium Hardness of your pool is to use test strips. Easy to use and affordable, these strips are soaked in a product that reacts with the calcium in the water when dipped in the pool.

Water hardness test kits

Test kits are a little pricier but are more accurate and allow you to analyse more pool parameters than test strips alone. These kits come with a reagent that is mixed with your pool water in a test tube.

Digital TH testers

Another option is to go for a digital pool tester. These testers are more expensive once again but can be used to test other parameters such as chlorine and bromine levels. Digital testers are usually the weapon of choice for pool professionals. 

Sequestrants are designed to lower the hardness of your pool water by combining calcium and magnesium ions to form molecules. These larger molecules are then held securely in the water making them unable to settle on your pool liner or equipment.

Liquid vs. tablets

These products can come in liquid or tablet form.  

• Solutions can be poured directly into your pool water. Simply follow the manufacturer's instructions to find the right dosage for your pool. Create movement in the water to distribute the product and switch off your filtration system for a few hours.

• Tablet versions are designed to be placed in the skimmer basket after each filter clean. Once again, the quantity (i.e. the number of tablets required) depends on the volume of the pool. This information should be supplied by the manufacturer.

How often to treat your pool water

How often you need to treat your pool against scale build-up depends on the TH of your water. Generally speaking, it's best to: 

• Carry out a first treatment as soon as you open your pool in spring.

• Repeat the treatment mid-season. You might even want to treat your pool twice if the water is particularly hard where you live.

• A final treatment should be carried out as you prepare your pool for winter.