Calcium (Ca)
The dominant cation in general hardness. What shrimp build moults from, what snails build shells from, and a macronutrient most planted tanks have plenty of.
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.
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.
How to test
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.
What high and low look like
Below 10 mg/L: invertebrate moult failures, snail shell pitting, plant leaf curl on new growth. 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.
How to fix
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.
