How much calcium chloride raises calcium by 10 ppm?
About 3.7 grams of calcium chloride dihydrate per 100 liters (13.9 g per 100 gallons). Ten ppm in 100 L is 1,000 mg of calcium; calcium is 40.08 g per mole, and a mole of the dihydrate weighs 147.014 g, so each gram of the powder is 27.3% calcium. The anhydrous salt is 36.1% calcium and needs only 2.77 g; hardware-store flake at 78% purity needs 4.7 g. Same salt, three strengths — which is why "a teaspoon per 25 gallons" advice is worthless unless it names the grade.
| System water | CaCl₂ dihydrate | Anhydrous | Flake (78%) |
|---|---|---|---|
| 10 gal · 38 L | 1.4 g | 1 g | 1.8 g |
| 20 gal · 76 L | 2.8 g | 2.1 g | 3.6 g |
| 30 gal · 114 L | 4.2 g | 3.1 g | 5.3 g |
| 40 gal · 151 L | 5.6 g | 4.2 g | 7.1 g |
| 55 gal · 208 L | 7.6 g | 5.8 g | 9.8 g |
| 75 gal · 284 L | 10.4 g | 7.9 g | 13.4 g |
| 100 gal · 379 L | 13.9 g | 10.5 g | 17.8 g |
| 125 gal · 473 L | 17.4 g | 13.1 g | 22.3 g |
| 150 gal · 568 L | 20.8 g | 15.7 g | 26.7 g |
| 200 gal · 757 L | 27.8 g | 21 g | 35.6 g |
| 300 gal · 1,136 L | 41.7 g | 31.4 g | 53.4 g |
Per 10 ppm of rise; multiply for your target. Grams, because calcium chloride is hygroscopic and its bulk density varies — weigh it. Dissolve in RO water and add slowly to high flow, away from the alkalinity dose. Keep daily rises to about 20 ppm.
Dihydrate, anhydrous, flake: which calcium chloride do I have?
Dihydrate (CaCl₂·2H₂O) is what the reef brands sell as "calcium chloride" — free-flowing white granules or prills, and the form in Randy Holmes-Farley's DIY two-part recipe. Anhydrous (CaCl₂, sold as Peladow and similar) is denser in calcium and gives off real heat when it dissolves; add it to water, never water to it. Flake ice-melt (Dowflake and its cousins) is mostly dihydrate at 77–80% with the rest water, and it works fine if it's a food- or pharma-grade product without anti-caking or anti-corrosion additives — read the SDS, and skip anything that lists them. Whatever you have, the calculator's three modes are the same formula with a different molar mass and purity plugged in.
Coral skeleton is calcium carbonate, built one calcium ion to one carbonate. Seawater carries about 420 ppm of calcium and only about 7 dKH of alkalinity — and in the units that matter for building skeleton, that's roughly twenty times more calcium than carbonate. So a day of growth that drops alkalinity a full 1 dKH takes only 7.2 ppm of calcium with it. Your alkalinity test swings daily; your calcium test looks stuck. It isn't stuck — it's just drawing on a much bigger reservoir, which is why the daily driver is the alkalinity dose and calcium is a weekly correction.
How fast can I raise calcium?
About 20 ppm a day is the usual advice — less because corals mind (they mostly don't) than because a concentrated calcium chloride dose landing near a concentrated carbonate dose precipitates calcium carbonate on the spot, clouding the water and wasting both. A 100 gallon system at 365 ppm aiming for 420 needs 55 ppm — 76.4 g of dihydrate — and the calculator makes that 3 days of 18.3 ppm, 25.5 g each, added slowly to strong flow at a different time of day from your alkalinity. Change the ceiling if you have a reason; the default is the cautious one.
How to mix and add calcium chloride
Weigh it — a kitchen scale that reads to a gram is the single most useful piece of reef equipment under $15 — then dissolve it in RO/DI water before it goes anywhere near the tank. Undissolved granules settling on rock or sand make a hot, concentrated spot that precipitates carbonate and can burn tissue. The calculator's "as a solution" line uses about 74 g per liter, roughly Randy Holmes-Farley's dilute recipe: strong enough that a day's dose is a small pour, dilute enough that it stays clear in the jug. Add it slowly into a high-flow area — a return chamber, a powerhead's outflow — and not within an hour of the alkalinity dose; calcium meeting concentrated carbonate at the same spot makes chalk, not coral. Anhydrous calcium chloride gets genuinely hot as it dissolves: add the salt to the water, never the water to the salt, and give it a few minutes to cool.
If the water clouds after a dose, that's precipitation — too much, too fast, or too close to the alkalinity addition — and it means part of what you paid for is now dust on the rocks. Cut the daily step, split it across two additions, and retest tomorrow before adding more; the cloud clears on its own and does no lasting harm.
Calcium won't stay up? Check magnesium first
If you keep dosing calcium and it keeps sinking back while alkalinity is fine, the usual culprit is magnesium below about 1250 ppm. Magnesium is what keeps calcium and carbonate from precipitating on every warm pump and heater in the system; run it low and your additions rain out as calcium carbonate before the corals get them. Enter your magnesium in the calculator and it flags this. The order is always the same: magnesium to 1250–1350, then alkalinity stable, then calcium — which, once the first two are right, mostly looks after itself.
If you dose kalkwasser for alkalinity, you're already adding 7.2 ppm of calcium per dKH — the same 1:1 ratio corals consume, which is why kalk-only systems rarely need a separate calcium dose. A balanced two-part does the same by design. Reach for straight calcium chloride when the calcium test genuinely drifts under 380 with alkalinity and magnesium in range, after a big water change with a low-calcium salt, or when setting up a new system.
What should reef calcium be?
Natural seawater is about 420 ppm; the consensus reef window is 380–450 ppm. Anywhere in it is fine; above about 500 you're spending money to precipitate carbonate, and below 380 calcification slows. Kits read to about ±10–20 ppm, so a "drop" of 15 ppm between tests is noise — dose to a trend, not a single reading. As always, the number to dose against is your true system volume, sump included; the calculator prefills it from your saved tank.