Chemistry Guide
Calcium Hardness: The Reading Everyone Ignores Until It Costs Them
Calcium hardness changes slowly, so it is easy to skip. But when it drifts out of range, the damage lands on the most expensive parts of your pool: the plaster, the heater, and the salt cell.
The target range
Keep calcium hardness at 200 to 400 ppm for most pools. Plaster and concrete pools should sit in the middle to upper part of that range, since the water will pull calcium from the surface itself if the level is low. Vinyl and fiberglass pools can run toward the lower end.
Test monthly. Unlike chlorine and pH, calcium barely moves week to week. It rises slowly through evaporation (water leaves, calcium stays) and through calcium hypochlorite shock, and it drops only when you add fresh water.
Too Low (under 150 ppm)
Water wants calcium and will take it from wherever it can. In plaster pools that means etching, pitting, and a rough surface. Low calcium water is also corrosive to metal parts, heat exchangers, and grout.
Common causes: fill water from soft municipal supplies, heavy rain dilution, frequent draining.
Too High (over 400 ppm)
Excess calcium comes out of solution as scale: white crust on the waterline and tile, deposits inside your heater that wreck its efficiency, and buildup on salt cells that shortens their life. High calcium also contributes to cloudy water.
Common causes: hard fill water, years of evaporation concentration, regular cal hypo shocking.
How to raise calcium hardness
Add calcium chloride, sold as calcium hardness increaser. As a rule of thumb, about 1.25 lbs per 10,000 gallons raises calcium hardness by 10 ppm. Pre-dissolve it in a bucket of water and pour it around the pool with the pump running. It generates heat when dissolving, so add it to the bucket slowly and never add water to the chemical.
Raise it in stages. Add enough for half the increase you want, circulate overnight, retest, then finish the job.
How to lower calcium hardness
Like stabilizer, calcium has no chemical that removes it. The practical fix is partial drain and refill with lower-calcium water. If your tap water is also hard, filling through a portable water softener or using trucked-in water are the remaining options.
While calcium is high, be strict about pH. Scale forms much faster at high pH, so keeping pH at 7.2 to 7.6 buys you time and protects the heater until you can dilute.
One habit that prevents most calcium problems
Know what is in your fill water. Test your tap water once for calcium hardness. If it is already 300+ ppm, avoid cal hypo shock (use liquid chlorine instead) because your pool will only ever drift upward. If your tap water is soft, expect to add calcium each spring and after heavy rain seasons.
What You Will Need
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