The Skeleton Budget

Alkalinity got the "heartbeat" title because it moves fast. But alkalinity is only half of a molecule. Every unit of skeleton your corals build is calcium carbonate — one calcium ion bonded to one carbonate ion — which means calcium is being consumed in lockstep with alkalinity, every hour, in a fixed ratio set by chemistry rather than opinion. And standing behind both of them is magnesium, the least glamorous and most misunderstood of the big three, quietly deciding whether the other two are even allowed to stay dissolved.

This chapter is the household budget for skeleton-building: what each ion does, what it costs, and how to keep the books balanced.

Calcium: The Big, Slow Reservoir

Target: 420 ppm, with 400–450 as the working range. Natural seawater sits around 412.

The first thing to understand about calcium is scale. At 420 ppm, your water holds an enormous working reserve compared to alkalinity's thin buffer. Run the comparison: a day of coral growth that knocks a meaningful 0.5 dKH off your alkalinity removes only about 3.6 ppm of calcium — a change most test kits cannot even resolve. Same consumption event, wildly different visibility.

This asymmetry explains almost everything about how the two parameters behave in practice:

• Alkalinity swings; calcium drifts. You test alk twice a week because it can move meaningfully in days. Calcium testing every week or two is plenty, because even heavy consumption takes weeks to move it far.
• Calcium problems are chronic, not acute. A tank almost never crashes from a calcium excursion; it slowly stalls as calcium grinds down toward 350 over a couple of months of un-dosed growth.
• Calcium is the lagging indicator of a dosing imbalance. If your two-part halves are mismatched to your tank, alkalinity tells you within days; calcium confirms it within weeks.

Within 400–450, precision matters little. Corals calcify essentially as well at 400 as at 450. What you should not do is chase supernatural numbers — pushing calcium past 500 buys no growth and starts flirting with abiotic precipitation, which we will meet shortly.

The Balanced Ratio: One Molecule, Two Ledgers

Because skeleton is CaCO3, consumption of calcium and alkalinity is stoichiometric — locked in a fixed ratio. In hobby units:

For every 1 dKH of alkalinity consumed, about 7 ppm of calcium is consumed with it.

(For the curious: 1 dKH = 0.357 meq/L of carbonate demand, which pairs with 0.179 mmol/L of calcium, and 0.179 mmol × 40.08 g/mol ≈ 7.2 ppm. The hobby rounds to 7 and loses nothing.)

This single ratio is the most useful piece of arithmetic in reef chemistry, because it lets you audit your tank:

• If alk and calcium are falling in ratio — say, 1 dKH and 7 ppm over the same stretch — that is clean, balanced coral growth. Dose both halves of a balanced supplement and you are done.
• If alk falls but calcium doesn't move, relax: given calcium's huge reservoir, that is usually just measurement resolution, and it is still probably balanced growth.
• If alk crashes while calcium rises, you have a dosing imbalance — you are overdosing the calcium half or under-dosing the alkalinity half.
• If both nosedive together faster than your corals can explain, suspect abiotic precipitation — the failure mode magnesium exists to prevent, and the subject of the next section.

Balanced two-part dosing solutions are formulated around exactly this ratio: dose equal amounts of each part and you replace calcium and alkalinity in the same proportion corals remove them. That is the entire elegance of two-part, and why the next dosing chapter can be so formulaic.

Magnesium: The Forgotten Gatekeeper

Target: 1300 ppm, range 1250–1350. Natural seawater: about 1285.

Magnesium is the third most abundant ion in seawater after sodium and chloride — ten times more plentiful than calcium — and corals do incorporate a little into their skeletons. But its day job is stranger and more important: magnesium is the reason your water can hold "too much" calcium and carbonate at the same time.

Here is the chemistry, kept honest but plain. Seawater at reef parameters is actually supersaturated with calcium carbonate — thermodynamically, CaCO3 "wants" to precipitate out as crystals until calcium and alkalinity both fall to far lower levels. The reason it doesn't is magnesium. Because the magnesium ion is chemically similar to calcium, it muscles into forming calcium carbonate crystals and gets incorporated where calcium should be — and its presence poisons the crystal surface, stopping orderly growth. Every incipient crystal gets capped before it can grow. The supersaturation holds, indefinitely, like a card tower nobody is allowed to touch.

Let magnesium sag to 1100 and the guard walks off duty. Now calcium carbonate begins precipitating abiotically — snowy dust on pump impellers and heaters, a crust in high-flow areas — and you experience it as the most maddening symptom in reef chemistry: alkalinity that will not hold no matter how much you dose. You raise alk to 8.5; two days later it is 7.4; you dose harder; the precipitation accelerates, because higher alk pushes supersaturation further. Reefkeepers in this loop have blamed their test kits, their salt, their corals, and their sanity. The answer was the third number they weren't testing.

Hence the rule that should be tattooed inside every dosing chapter: when alkalinity misbehaves, test magnesium before you change anything. And its corollary for corrections: fix the big three in order — magnesium first, then calcium, then alkalinity — because the later ones will not stay fixed until the earlier ones are right.

Day to day, magnesium is the laziest of the three: consumption is slow, and regular water changes with a quality reef salt mix often replace it entirely. Test it every week or two — a reef master test kit covers calcium and magnesium alongside everything else — and dose only what the number justifies.

Correction Math, Done Calmly

When a big-three parameter is genuinely low, the correction follows the same calm procedure every time:

1. Retest first. Magnesium titrations especially are fiddly, and a single wild reading is more often technique than truth.
2. Compute the deficit. Target minus actual. Example: magnesium at 1180, target 1300 — deficit 120 ppm.
3. Compute the dose from your product's label. Every reputable supplement states something like "X ml raises Y gallons by Z ppm." Scale linearly to your actual water volume — displacement by rock and sand means a "75-gallon system" often holds 60 gallons of water. When in doubt, assume less water; undershooting is always recoverable.
4. Spread it out. Magnesium corrections can go in over 2–3 days; calcium over a few days; alkalinity — the sensitive one — at no more than 1 dKH per day, as established in the alkalinity chapter.
5. Retest after 24 hours and let the measured result, not the arithmetic, decide the next dose.

The pattern to internalize: arithmetic proposes, the test kit disposes. Every calculator and label rate is an estimate standing on your salinity, your true water volume, and your kit's accuracy — three things that are never quite what you assume. Dose, measure, adjust — never dose the full theoretical correction blind, and never dose a second time before the first correction has had a day to register on a retest.

FAQ

My calcium is high and alkalinity is low (or vice versa). Which do I fix?

Fix the low one, gently, and stop supplementing the high one until the pair re-converges. High-calcium/low-alk is by far the more common state (imbalanced dosing, or heavy use of calcium-rich media), and the fix is simply dosing alkalinity alone — respecting the 1 dKH/day ceiling — while calcium coasts down through consumption. Check magnesium first, as always; imbalances hold longer when the gatekeeper is low.

Do water changes keep calcium and magnesium up without dosing?

In lightly stocked tanks, genuinely yes — a 10% weekly change with a reef-grade salt mixing at 430 Ca / 1350 Mg continuously nudges both toward those values. As stony coral mass grows, consumption eventually outruns any sane water-change schedule, and the crossover usually announces itself through alkalinity first. Water changes then remain your trace-element and reset mechanism while dosing carries the big three.

Is more magnesium a performance booster?

No. Magnesium is a threshold parameter, not a throttle: below range it causes real, cascading problems; within range, raising it further does nothing measurable for growth or color. Chasing 1450+ wastes supplement and, at extremes, stresses livestock. Keep it 1250–1350 and spend the attention on alkalinity stability instead.