Calcium hardness

Raising it

Calcium chloride. About 1.3 lb raises hardness 10 ppm in 15,000 gallons. Dissolve it in a bucket first and pour it in the deep end. It gives off heat as it dissolves, so don't hold the bucket against you.

Lowering it

You can't chemically remove calcium. The only ways down are replacing water:

• Partial drain and refill, if your fill water is softer than the pool
• Reverse osmosis on site, which is common where water is expensive or the tap is very hard

In a market where the tap runs 500 ppm, draining and refilling barely helps. That's where RO earns its cost.

Calcium hardness is how much dissolved calcium the water is carrying. Plaster is made of calcium, and water low in calcium will pull it out of the surface to get some. That's etching, and it doesn't grow back.

Too much calcium goes the other way. It comes out of solution and deposits on tile, heat exchangers and salt cells.

Vinyl and fiberglass

There's no plaster to protect, so those pools can run lower, around 150–200 ppm. They still shouldn't run near zero, because very soft water is aggressive toward the metal parts of the equipment.

Where the calcium comes from

Mostly the fill water. In Phoenix and Las Vegas the tap arrives at 300–500 ppm and sometimes higher. Every top-up adds more, and evaporation leaves it behind, so hard-water pools climb all season without anyone adding anything.

Cal-hypo shock adds calcium too. On a hard-water pool, or any salt pool, use liquid chlorine instead.

Testing cadence

Monthly is enough for most pools. Test it sooner if you see scale forming, if a salt cell is losing output, or the first time you take on a pool.

Calcium and LSI

Calcium is one of the five LSI inputs, and it's the one you can't change on the day. So on a high-calcium pool you manage the LSI with the levers you do have: hold pH at 7.4 and TA at 80–100. See reading the LSI.