Calcium and Magnesium in Freshwater Aquariums

How calcium and magnesium make up water hardness, how to convert mg/L as Ca to mg/L as CaCO3 and dGH, their roles for fish, moulting crustaceans and snails, and how RO water is remineralised.

What hardness actually measures

In water science, hardness is the amount of dissolved calcium (Ca2+) and magnesium (Mg2+). These ions are picked up as groundwater moves through limestone, chalk, gypsum and other mineral-bearing rock, so the hardness of a supply depends largely on local geology. The US Geological Survey classifies water as soft at 0–60 mg/L, moderately hard at 61–120 mg/L, hard at 121–180 mg/L and very hard above 180 mg/L, all expressed as calcium carbonate (CaCO3). Hardness linked to bicarbonate is called temporary because boiling removes it, while hardness from calcium and magnesium sulfates and chlorides is permanent. General GH/KH concepts are covered in the gh-kh-hardness-guide, and the bicarbonate–carbonate equilibrium in the carbonate-system-chemistry guide; this article looks at the two ions themselves.

Units: "as Ca" versus "as CaCO3" and dGH

Laboratory reports and scientific papers use two different conventions. A calcium value "as Ca" (or Ca2+) is the mass of the calcium ion itself. Hardness "as CaCO3" expresses all hardness-forming ions as an equivalent mass of calcium carbonate. The two numbers are not interchangeable. The US EPA gives the conversion as: hardness (mg/L as CaCO3) = 2.497 × Ca (mg/L) + 4.118 × Mg (mg/L). Because the magnesium factor is larger, one milligram of magnesium contributes more hardness than one milligram of calcium, and a single total-hardness figure does not show how it is split between the two ions.

The German degree (dGH or °dH) is defined as 10 mg/L of calcium oxide (CaO), which equals 17.848 mg/L as CaCO3. One millimole of hardness ions per litre corresponds to 100.09 mg/L as CaCO3, or 5.608 dGH.

Worked example (calculated here with the EPA equation and the dGH definition above): water with 20 mg/L Ca and 5 mg/L Mg has 2.497 × 20 + 4.118 × 5 ≈ 70.5 mg/L hardness as CaCO3.
Dividing by 17.848 gives about 3.9 dGH for the same water.

Roles in fish

Magnesium is mainly an intracellular ion: ATP must be bound to Mg2+ to be biologically active, and more than 300 enzymes depend on it, including those involved in protein synthesis and energy metabolism. Calcium is the dominant hardness ion in most natural freshwaters. A 2025 study on Nile tilapia (Oreochromis niloticus) notes that a calcium level of 25–100 mg/L as Ca2+ is often advised in aquaculture, that hardness can reduce the toxicity of heavy metals in some species, and that extremely hard water can harm fish physiology and reproduction. In goldfish (Carassius auratus) exposed to zinc, harder water reduced zinc accumulation, stress markers and liver cell death.

Effects depend on the species. A study cited for early-life stages recommends keeping hardness above 20 mg/L as CaCO3 for hatching and larval survival. When Poecilia reticulata and Betta splendens were kept at 150 (control), 320, 540 and 900 mg/L as CaCO3, guppy growth and fecundity were highest at 900 mg/L, whereas bettas showed significantly lower hatching rates and disturbed bubble nests at the same level.

Crustaceans and moulting

Crustaceans harden a new exoskeleton after each moult by depositing calcium carbonate in it. Their calcium demand changes through the moult cycle: net uptake is close to zero between moults and rises before and after ecdysis. Crayfish take up calcium from both food and water and can reabsorb it from the old shell. Freshwater crayfish also form gastroliths, mineral stores in the stomach that supply calcium for the next moult, an adaptation to the low calcium content of freshwater. In red claw crayfish (Cherax quadricarinatus) fed diets with 3–9% calcium carbonate, the carapace contained 20–23% calcium, and calcium-supplemented diets improved growth and moulting frequency.

Magnesium also matters. In Chinese mitten crab (Eriocheir sinensis), dietary calcium and magnesium interacted in their effects on growth, carapace hardness and moulting hormone levels, and the authors concluded that higher dietary calcium should be matched by more magnesium in the feed. More calcium in the water is not always better: high calcium hardness has been reported to lower survival of the freshwater prawn Macrobrachium rosenbergii.

Snails and shell building

Freshwater pulmonate snails take most of their shell calcium directly from the water rather than from food. In a 12-week recirculating-system trial, juvenile apple snails (Pomacea flagellata) reared at 500 mg/L calcium were significantly longer and heavier and had harder shells than snails at 300 or 243 mg/L. The species' native lake has about 325 mg/L calcium. The same paper gives a pH requirement of 6.5–8.0 and notes that the shell begins to erode at pH 5.0 or lower.

The calcium-to-magnesium ratio

Calcium and magnesium add to the same hardness value but are not interchangeable in physiology. For example, Mg2+ ions block some Ca2+ channels in cell membranes. No scientific, extension or regulatory source consulted for this guide gives an optimal Ca:Mg ratio for freshwater aquariums, so specific ratios are not stated here.

Remineralising RO water

Reverse osmosis lowers calcium, magnesium, sulfate, nitrate and many other dissolved substances, leaving water with almost no hardness. Drinking-water desalination plants stabilise their product water to prevent corrosion, usually with lime or caustic soda. One plant dissolves calcite (limestone) with sulfuric acid and reaches 40–46 mg/L calcium.

For toxicity testing, the US EPA describes a standard synthetic freshwater made by adding four reagent-grade salts to deionised water: sodium bicarbonate, calcium sulfate dihydrate, magnesium sulfate and potassium chloride. The calcium sulfate is stirred into a separate litre until it dissolves and then added to the rest, and the mixture is aerated for 24 hours before use.

Soft: 48 mg/L NaHCO3, 30 mg/L CaSO4·2H2O, 30 mg/L MgSO4, 2 mg/L KCl → hardness 40–48 mg/L as CaCO3, alkalinity 30–35 mg/L as CaCO3, pH 7.2–7.6.
Moderately hard: 96, 60, 60 and 4 mg/L of the same salts → hardness 80–100 mg/L, alkalinity 57–64 mg/L as CaCO3, pH 7.4–7.8.

In this recipe, the calcium and magnesium sulfates supply the hardness (sulfate hardness is the permanent type), while sodium bicarbonate contains no calcium or magnesium and supplies the alkalinity. Calcium chloride dissolves quickly because it is highly soluble, whereas calcium carbonate dissolves poorly. Softener-treated water and rainwater, which raise different issues, are covered in their own guides.