Calcium and Magnesium Cycles

For a high-level tour of how every element moves through the tank, start with Nutrient Cycling. Calcium is the element that builds shells — and, in a small closed tank, the one that can run out from under the animals that depend on it.

Calcium is a one-way material, not a recycled nutrient

Most nutrients go around in a loop: a plant takes up nitrogen, an animal eats the plant, the animal dies, bacteria break it down, and the nitrogen is released for the next plant. Calcium doesn't really work that way. It is a building material — the stuff of snail shells and shrimp exoskeletons — and once an animal locks it into a shell it does not come straight back. A dead snail's shell sits on the bottom and dissolves only grudgingly, and only if the water chemistry happens to favour dissolution. In many tanks it does not.

That makes calcium unusual: in a small system the handful of shell-building species can draw dissolved calcium down faster than anything replenishes it. Over months, calcium can become the limiting factor for snails and shrimp even when every other nutrient is plentiful — a slow squeeze that shows up as soft, pitted, or poorly-formed shells long before anything else looks wrong. Magnesium, calcium's partner in general hardness, tells the opposite story: no organism in the model touches it, so it just sits there. The two start together and drift apart, and watching them separate is one of the clearest signs of active shell-building in a simulation.

Where calcium comes from

Calcium enters the tank dissolved in the source water, and for most setups that starting stock is all there is. When you set up a tank by its general hardness rather than spelling out calcium and magnesium separately, the model splits the hardness the way typical tap water does — roughly two-thirds calcium, one-third magnesium by count — so a moderately hard tank at six degrees of hardness starts with around 28 mg calcium and 9 mg magnesium per litre. You can also state either one explicitly to override that split.

The only thing that actually adds dissolved calcium during a run is a calcareous substrate slowly dissolving. Crushed coral and aragonite sand are nearly pure calcium carbonate, and when the water is hungry for carbonate (see below) they dissolve to release calcium back into solution — along with the alkalinity and dissolved inorganic carbon that come with it. This is the aquarist's classic trick for keeping hard water hard in a tank full of snails and shrimp: the substrate acts as a slow-release calcium and alkalinity reservoir that tops up what the animals remove. Without a calcareous substrate, though, there is no calcium source at all — once it is consumed, it is gone. The two coral and aragonite presets and their dissolution rates are detailed in the Parameter Reference.

Where calcium goes

Two animals spend the tank's calcium, and they spend it differently.

Snails build their CaCO₃ shells as they grow, depositing shell in step with new body tissue. When a snail isn't growing, it isn't building shell. Soft water makes this hard: below roughly three degrees of hardness, Physa and similar bladder snails can't lay down shell fast enough, their growth is impaired, and the population struggles — exactly the real-world observation that snails do poorly in very soft water (Dillon 2000).

Shrimp spend calcium a different way. Dwarf shrimp like Neocaridina don't grow a shell continuously — they molt, shedding their old exoskeleton and hardening a new one on a regular cycle whether or not they are growing. So their calcium demand is a maintenance cost, paid at every molt. In soft water the cost bites twice over: shrimp molt less often, and each molt becomes more dangerous, because the soft-shelled window right after a molt leaves them vulnerable until the new shell hardens. The model represents the shrimp's exoskeleton as pure calcium carbonate, a slight simplification that omits the chitin and phosphate fractions of a real shell.

In both cases, building a shell consumes alkalinity and dissolved inorganic carbon along with the calcium, in fixed proportion — every unit of shell carbonate takes two units of alkalinity with it. That alkalinity cost is small for a few animals but real for a dense population, and it is one of the quieter ways a heavily-stocked tank can drift its pH downward over time.

When a calcifier dies, or a shrimp sheds its shell, the calcium does not flush straight back into the water. The shell or exuviae becomes solid carbonate sitting on the substrate, where it dissolves on the same slow, chemistry-dependent terms as any other calcareous material. So shell calcium is eventually recyclable — but only slowly, and only if the water is undersaturated enough to dissolve it.

The saturation state: will carbonate dissolve or precipitate?

Whether solid calcium carbonate dissolves or forms comes down to a single thermodynamic quantity — the calcite saturation state, written Ω. Think of it as how "full" the water is with respect to carbonate:

• When the water is undersaturated (Ω below 1), it is hungry for carbonate, and solid carbonate — substrate, dead shells — dissolves. Soft water, low pH, and high dissolved CO₂ all push toward dissolution.
• When the water is supersaturated (Ω above 1), it is over-full, and calcium carbonate tends to precipitate out. High pH and high calcium push this way. A burst of intense daytime photosynthesis can drive pH up past 8.5, tip the water into supersaturation, and precipitate calcium carbonate right out of the water column.
• At Ω of exactly 1, the water is in balance and nothing happens.

Two details matter for aquarists. First, aragonite — the mineral of coral and marine sand — is meaningfully more soluble than ordinary calcite, which is why coral-based substrates dissolve more readily and buffer better in soft, acidic water (Plummer & Busenberg 1982). Second, the saturation state is always reported in the simulation output, even when you haven't enabled carbonate precipitation and dissolution, so you can always see which way the water wants to go even if you aren't modelling the kinetics. The mineral solubility constants live in the Parameter Reference.

How calcium moves: the picture

Calcium runs a small loop between dissolved form, living shell, and solid substrate carbonate:

            source water  (starting calcium stock)
                   │
                   ▼
        ┌────► dissolved calcium ◄──────────┐
        │            │                       │
   snail shell   shrimp molt          calcareous substrate
   building      (every cycle)        dissolves (water is
        │            │                 carbonate-hungry)
        ▼            ▼                       │
   [locked into a living shell]             │
        │            │                       │
   on death     shed exoskeleton            │
        │            │                       │
        ▼            ▼                       │
        solid carbonate on the substrate ───┘
                   ▲
                   │
        precipitates out when the
        water is carbonate-over-full
        (high pH from photosynthesis)

The loop only closes slowly, and only when conditions allow. In a tank with calcifiers but no calcareous substrate, the dissolution arm is effectively shut, and calcium drains steadily into shells with little coming back.

Magnesium: same start, different fate

Calcium and magnesium come in together, split out of the same hardness setting. From that moment their paths diverge completely. Calcium is consumed by shell-builders, pulled down by precipitation, and topped up by dissolving substrate. Magnesium does none of this — no organism in the model consumes it, no mineral removes or adds it, and it has no effect on alkalinity or pH. Real ecosystems do use a trickle of magnesium (it sits at the heart of every chlorophyll molecule), but in a small tank that draw is negligible — calcium runs out long before magnesium would. Magnesium is therefore a conservative pool: it holds dead steady at its starting value.

The practical upshot is that in any tank with active calcifiers, the magnesium-to-calcium ratio in the water climbs steadily as calcium is drawn down while magnesium holds firm. General hardness falls, but the fall is driven entirely by the calcium side. That widening gap is the signature of biological calcification at work.

How calcium shapes hardness, alkalinity, and pH

Calcium and magnesium together set general hardness. Because only calcium is consumed, hardness in a calcifier tank declines over time — but more gently than calcium alone would suggest, because the steady magnesium contribution cushions the drop. In soft water this decline can cross the thresholds where snails' shell growth falters and shrimp molting turns risky, tipping a marginal tank into trouble. A calcareous substrate breaks the decline: as fast as the animals remove calcium, dissolution replaces it, and hardness stabilizes at the level the substrate's saturation balance sets.

Calcium's effect on alkalinity is its loudest interaction with the rest of the tank. Every carbonate reaction — a snail building shell, carbonate precipitating, substrate dissolving — moves two units of alkalinity for each unit of carbonate. So a dense snail population steadily eats alkalinity and can measurably lower pH over time, while a calcareous substrate works the other way, dissolving in acidic water to release alkalinity and buffer the pH back up. This makes calcium one of the more dynamically interesting elements in the model despite being needed by only a couple of species. Magnesium, by contrast, is inert with respect to the carbonate system and never touches alkalinity at all.

How calcium and magnesium compare to the other nutrients

Conservation

Calcium is conserved across the tank, counting dissolved calcium, the calcium locked in living shells and exoskeletons, and the solid carbonate on the substrate. There is no gaseous escape the way nitrogen has; the only thing that changes the total is a water change. Because shell calcium is tracked implicitly with biomass rather than as a separate pool, an exact accounting means adding the substrate carbonate to the calcium implied by the living calcifiers. Magnesium is conserved even more simply — with no sinks or sources of any kind, its total never moves except through water changes.

Further reading

• The Carbonate System — how calcium ties into carbonate speciation, pH, and alkalinity
• Consumers — shell-building snails and molting shrimp, the animals that spend the tank's calcium
• Stability and Failure — how calcium depletion and pH swings shape a tank's long-term health
• Parameter Reference — every rate constant, stoichiometric ratio, and stress threshold behind this page, with citations

Key references

• Berner, R.A. (1975). The role of magnesium in the crystal growth of calcite and aragonite from sea water. Geochimica et Cosmochimica Acta 39, 489–504.
• Dillon, R.T. (2000). The Ecology of Freshwater Molluscs. Cambridge University Press.
• Morse, J.W. & Arvidson, R.S. (2002). The dissolution kinetics of major sedimentary carbonate minerals. Earth-Science Reviews 58, 51–84.
• Plummer, L.N. & Busenberg, E. (1982). The solubilities of calcite, aragonite and vaterite in CO₂–H₂O solutions between 0 and 90 °C. Geochimica et Cosmochimica Acta 46, 1011–1040.
• Stumm, W. & Morgan, J.J. (1996). Aquatic Chemistry, 3rd ed. Wiley-Interscience.