How to raise calcium in a reef tank

Short answer: dose a calcium supplement — calcium chloride, or a commercial calcium product — in an amount calculated for your water volume, and raise it no more than about 20 ppm per day. But before you do: calcium is the parameter least likely to actually be your problem. Here's how to raise it, and how to be sure it's the thing that needs raising.

First, check it's really low

Calcium behaves differently from alkalinity. There's a lot of it in your water (~420 ppm), and it's consumed slowly relative to how much is there — so it tends to sit stable. Two things follow:

• A single "low" calcium reading is often kit error. Hobby calcium kits can be off by 10–20 ppm. Retest before you dose.
• If calcium is genuinely low but alkalinity is fine, magnesium is often the real story. Low magnesium makes it hard to hold calcium up. Check magnesium first — raising it may fix calcium without dosing any.

If it checks out low across a couple of clean tests, then raise it.

How much to add

Plain calcium chloride is the cheap, reliable option: about 7 g of calcium chloride (dihydrate) raises 100 litres (26 US gal) by roughly 20 ppm. Commercial calcium products and two-part systems work the same way. Use the exact figure for your tank:

The calculator uses your net water volume and holds you to the safe daily limit.

The safe daily limit

Raise calcium by no more than about 20 ppm per day. As with alkalinity, the concern is supplement impurities going in fast, not calcium itself. A big correction gets split over several days — the calculator does that for you.

How to dose it safely

• Never dose calcium and alkalinity at the same time or in the same spot — together they can precipitate out (the "snowstorm") and strip both. Space them apart.
• Correct magnesium first if it's low, or the calcium won't hold.
• Add to a high-flow area; retest after a day.

Raising vs. maintaining

Because calcium moves with alkalinity, the steady way to keep it up is usually to dose your two-part or calcium supplement to match consumption — not to correct calcium on its own. If your numbers keep drifting, the app works out your actual consumption and tells you the dose that holds all three together.

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