Calcium (Ca)
The dominant cation in general hardness. What shrimp moult with, what snails build shells from — and a macronutrient planted tanks rarely lack.
In a freshwater tank, calcium wants to sit between 10 and 30 mg/L. Below 10 mg/L, shrimp moults start failing and snail shells pit; going up, nothing dramatic happens until roughly 100 mg/L, and even then the problem is the very hard water that number signals rather than the calcium itself. If you only know your GH: each 7.1 mg/L of calcium contributes one degree, so the target band is about 1.5–4 °dGH of calcium alone.
What it is
Calcium is the most abundant divalent cation in fresh water, and the largest contributor to general hardness. In planted-tank fertilisation it's a macronutrient; in invertebrate keeping it's structural — every moult of a shrimp and every layer of a snail shell pulls calcium directly from the water column.
It never travels alone, though. GH is calcium plus magnesium, so a GH reading tells you the mineral pool exists but not how it's split — two tanks at 6 °dGH can hold very different calcium levels depending on the source water.
Why it matters
Plants use calcium to build cell walls; a deficiency shows up first in the youngest leaves as crinkling or hooked tips. Shrimp need ~10 mg/L minimum to moult cleanly, and snails need it constantly to maintain shell integrity — soft, pitted shells almost always trace back to chronic low Ca.
In tap water from chalk or limestone aquifers, calcium is already at 30–80 mg/L. In RO-based tanks you control it through remineralisation.
Where it enters the tank
Three routes: tap water, remineralising salt — and the décor. Limestone, seiryu and other calcareous rock, coral sand and shell grit all dissolve slowly in soft, slightly acidic water; a tank whose calcium creeps upward between water changes without any dosing is being dosed by its rocks. A drop of vinegar on the dry stone settles it before it goes in — if it fizzes, it dissolves.
When calcium runs short
Below 10 mg/L: invertebrate moult failures, snail shell pitting, plant leaf curl on new growth.
In shrimp the sequence is predictable: a moult takes longer than it should, then one fails outright — the animal stuck half out of its old shell, showing the "white ring of death" behind the head. Fresh shells stay soft and papery for days instead of hours. Snails show it slower but more visibly: new growth at the shell lip comes in thin and translucent, and older whorls etch and pit — faster in acidic water, where low pH dissolves shell from outside while low Ca starves the rebuild from inside. Plants lock calcium into tissue they can't remobilise, so the damage sits in the newest leaves and stalled growing tips, while old leaves look fine — exactly the opposite pattern to magnesium deficiency. Fish are the least sensitive of the lot: calcium-poor water is simply very soft water, which only wears on hard-water species like livebearers. If shrimp are why you're reading this, the shrimp-safe parameters guide has the full moulting picture.
When it runs high
Above 100 mg/L on its own isn't toxic, but it usually means hard water that pushes other parameters (Mg, KH, pH) up alongside it.
Between 30 and 100 mg/L you're in drift territory, not danger: livestock is fine, but every extra 10 mg/L adds about 1.4 °dGH, and in tap-fed tanks KH and pH usually ride along. The costs are indirect. Soft-water fish stop breeding long before they look stressed — tetra and Apistogramma eggs simply fail to develop in hard water. A strongly skewed Ca:Mg ratio (around 10:1 or more) can push plants into magnesium deficiency even though both minerals are "present", because the two compete for uptake. And a white crust at the waterline shows what months of topping up evaporation with hard tap does: the water leaves, the calcium stays — top up with RO instead.
Raising and lowering it
Too low — Seachem Equilibrium, Salty Shrimp Mineral GH+ (for shrimp tanks), or directly dosed calcium chloride (CaCl₂). 1 g CaCl₂ per 100 L adds ~3.6 mg/L Ca. The dosing calculator handles the bottle-to-tank math. Too high — dilute with RO water; calcium binders exist but are rarely needed at hobby levels.
| Route | Effect | Notes |
|---|---|---|
| CaCl₂ (dry) | 1 g / 100 L ≈ +3.6 mg/L Ca | fast and clean; adds chloride |
| CaSO₄·2H₂O (gypsum) | 1 g / 100 L ≈ +2.3 mg/L Ca | dissolves slowly; adds sulfate |
| Crushed coral / aragonite in the filter | slow, self-limiting | raises KH and pH too |
| Cuttlebone | very slow | the lazy classic for snail tanks |
Move it in steps — shrimp moult on osmotic cues, and a sudden mineral jump can trigger a wave of premature moults; a few mg/L per day is plenty. Going down is only ever dilution: cut the change water with RO — a 30% change with pure RO removes about 30% of the excess, and the water-change impact tool shows the curve over several weeks. If the level rebounds between changes without dosing, check the hardscape, not the tap.
Testing without fooling yourself
Salifert Ca, JBL Calcium, or a calculated value from GH if you know your Ca:Mg ratio (tap water typically sits near 4:1 calcium-to-magnesium). The hardness converter handles the Ca/Mg split from a GH reading.
One caveat with the drop kits: most calcium titration tests were designed for reef tanks, where 400–450 mg/L is normal. Their per-drop resolution is often 10–20 mg/L — at freshwater levels that puts the entire target band inside one or two drops, so a single reading is coarse by construction. Two ways around it: double the water sample to halve the step size, or measure GH — cheap and fine-grained at the low end — plus one careful Mg test, and take calcium as the remainder. Calcium barely moves week to week in a stable tank; monthly is enough, plus a source-water test whenever your mix changes. It's a slow parameter, which makes it a logging parameter: one entry a month, and drift that would be invisible between two tests becomes an obvious line.
