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Calcium Hardness Scaling and Staining

It is important to be aware of the calcium hardness (CH) level of your water. The CH level is especially important for pools with surfaces below the water line that contain calcium, such as plaster, pebble, gunite, concrete, quartz, tile, or stone. But all pool owners need to control their CH level to some extent.

If there is too much calcium in the water, calcium scale can form on the pool and in the plumbing, and the water can become clouded with calcium dust. If there is too little calcium in the water, and you have a surface containing calcium (plaster / marcite), calcium from the pool surface can dissolve into the water, causing pitting and deterioration therby shortening the life of your finish.

The risk of calcium scale, clouding, and pitting depends not just on the calcium level, but also on the PH, total alkalinity, and several other factors. The Calcium Saturation Index (CSI) is a single number showing the overall risk for scaling or pitting. PH actually has the largest impact on the CSI, more than calcium does! If you allow your PH to get too high, there is a significant risk of calcium scaling.

You can use The Pool Calculator to find out your CSI. You can also use the similar, but somewhat less accurate, Langelier Saturation Index (LSI), which is easier to calculate by hand but is less precise.

To prevent scaling, you should keep your CSI below 0.6 at all times. Pools with surfaces containing calcium also need to have their CSI above -0.6 at all times to prevent pitting. Vinyl, fiberglass, and painted pools can safely have a significantly negative CSI. The CSI is very sensitive to PH changes. With surfaces containing calcium you normally try to balance your CSI fairly close to zero so that future PH swings will still leave the CSI within these bounds.

Calcium also serves other purposes. If you have a spa, it is best to keep the CH level at 150 or higher to reduce foaming. CH levels around 200-250 are recommended for fiberglass pools, to protect the gelcoat and somewhat reduce the severity of metal stains.

In many parts of the country, the fill water has high levels of CH. Combine this with high rates of evaporation in arid areas, and you can rapidly raise the CH level in the swimming pool. CH levels up to perhaps 600 (we feel) can be managed by reducing your TA and carefully keeping your PH relatively low, but the “accepted” levels are in the 200-400 ppm range, and this is not really advised.

The most widely used way to lower CH levels is to replace water (drain and refill), but that can be expensive and wasteful. If replacement water is expensive, or if you prefer to save water and get a better quality water, you can also use a reverse osmosis (R/O) water treatment to lower calcium. Reverse Osmosis treatments will also remove other contaminants from your pool (viruses, bacteria, CYA, nitrates, phosphates, salt, etc.) and will return “bottled water” to your pool. In these areas where this is an option, the water companies cannot provide anything remotely close to the quality of water you will get from a R/O treatment.

Calcium scale can be very difficult to remove, as it is extremely hard. Small patches of scale can be removed with a pumice stone and elbow grease! In mild cases, lowering your CSI to about -0.8 and brushing aggressively can sometimes remove scale. This can take weeks and you need to be extremely careful not to lower your pH too far, as you can damage copper heat exchange coils or other copper items in the pool. There are “no drain acid wash” treatments, and the heater (if any) must be bypassed for protection. The process is somewhat corrosive to metal items, and may leave a rougher surface than was previously existing.

The only reliable solution for removing calcium scaling is a full drain and acid wash, which is very hard on plaster surfaces (the general rule of thumb is that an acid wash will take roughly three years off of the life of a plaster finish). If at all possible, avoid doing an acid wash in most cases, in plaster pools. PebbleTec (or similar) pools can handle these, but the best action is to maintain your pool well enough that you do not need to consider having an acid wash performed!