There are two things you want to keep in mind about calcium in your pool or spa; first, calcium is your friend and second calcium is your enemy. “Wow, Jim, that is pretty conflicted,” you say, but the proper amount of calcium in your water with a proper water balance will keep your water looking clear and protect your pool surface.
The proper level for calcium in your pool will depend on the type of pool surface you have. Vinyl liner, painted and fiberglass pools will need at least 170 ppm of calcium in the water and levels up to 400 ppm are acceptable. Concrete pools will need a higher starting point of at least 225 ppm. If you have a new masonry finish in the pool you will need to pay very close attention to your water balance to protect the new surface from all types of complications once the pool is full.
Why do you need calcium in the pool water anyway? This is a question we are frequently asked. Calcium does several things for you; first it helps the filter be more efficient at doing its job to the best of its ability. If your calcium is too low you may have difficulty getting your pool to clear. The other reason it is important relates to the other mineral balance in your pool, total alkalinity. You will recall in Pool School 104, when we talked about total alkalinity, water wants a certain water balance and if it does not have that balance it will begin to satisfy it’s need by drawing chemicals, or mineral, out of whatever vessel it is being held in. How that translates to your pool will depend on the type of surface you have in the pool. For people who have vinyl liner pools it will pull plasticizers’ out of the vinyl making the liner brittle. I had a customer whose wife left him, he had purchased the pool for her and when she left he was over the pool and he turned it off. The pool sat unused for two years. Not only did he not ever touch the pool, he didn’t even look at the pool. Fast-forward two years, he has a new girlfriend and she is all about the pool so he figures he better get it going. He hires someone to come out and clean up the swamp. The problem is the chemistry has been so neglected that the liner has not only badly wrinkled from low pH and no alkaline minerals, the liner has turned so brittle that when you push a brush across the folds of the wrinkles the liner cracks, because it is brittle from low calcium. Needless to say he had to replace the liner.
So what happens if the calcium is too high? Calcium in and of itself is not a problem; it is when you combine it with other minerals in the water that it becomes an issue. Temperature also is a factor in overall balance of minerals. We just talked about what happens if the water isn’t satisfied with the mineral balance but the other side of the scale is when there are too many minerals in the pool water. What happens is the water has to release something and the first thing it gives up is calcium. Calcium will deposit it on the surface of the pool making the surface feel like sandpaper. Think of it like over-eating, if your stomach can’t hold any more food it will, not to be graphic, get rid if it. Not a pleasant experience, and while removing calcium from the surface of the pool is not exactly the same thing as what your stomach just did, it is still an arduous, un-pleasant task. Removing calcium from the surface of the pool will usually take about 2 to 4 weeks and require the addition of a chemical specifically designed to re-dissolve the calcium back into the pool water and hold it there. It will also require you to brush the surface of the pool everyday in order to facilitate the scale removal.
The other unique thing about low calcium is that it can make the water have a slight green tint to it. We usually see this in pools where the calcium level is below 100ppm. The water will be clear, it just will have a green tint and shocking does not clear the green tint. I once had a commercial customer that was an hour ride from my store, and they called me and told me the pool was clear but green, and when they shock the pool it doesn’t go back to blue. So I told them I needed a water sample to resolve the problem. They made 2 more attempts to solve the problem themselves, one of which involved the chemist from the local paper mill. The paper chemist had declared the water in perfect balance, but the pool water was still green. They called me again, and again I asked them to bring me a sample of the pool water, since they had not had any luck so far they finally relented and brought the sample to me. When I tested the water I discovered that the calcium hardness was at 30ppm, way to low, I had them add a sufficient amount of calcium and, low and behold, the pool was blue again. While the paper mill chemist was an expert at paper chemistry he really did not understand swimming pool chemistry.
This concludes this class on calcium hardness, next class will be on stabilizer so make sure you are on time, late arrivals will be sent to the principals office!