Calcium, magnesium and GH for aquarium plants
Short answer
GH counts calcium and magnesium together. One German degree is 0.178 mmol/L, which is 7.15 mg/L of calcium or 4.33 mg/L of magnesium [3, 4]. Plants need both: calcium builds cell walls and magnesium sits at the center of chlorophyll [1]. A GH test cannot tell you the split, so the dosing methods target calcium and magnesium separately.
What GH measures
General hardness (GH) is the concentration of divalent “hardness” ions in the water, in practice calcium and magnesium. Aquarium kits report it in German degrees, where one degree is defined as 10 mg/L of calcium oxide equivalent, 17.848 mg/L as CaCO3, or 0.1783 mmol/L of hardness ions [3].
Because a degree is a count of ions, not a mass, the same degree can be made of different amounts of each metal. Multiplying 0.1783 mmol/L by the atomic weights [4]:
- 1 °dH of calcium = 7.15 mg/L Ca
- 1 °dH of magnesium = 4.33 mg/L Mg
Aquasabi’s wiki gives 7.144 mg/L Ca and 4.3556 mg/L Mg per degree [5]; the calcium figure agrees and the magnesium figure is about half a percent higher than the atomic-weight calculation. Either way, the conclusion holds: a GH test cannot tell you how much of the hardness is calcium and how much is magnesium. Separate Ca and Mg tests, or a water report from your supplier, are needed for that.
Why plants need both
Calcium and magnesium are secondary macronutrients, absorbed as the Ca²⁺ and Mg²⁺ ions [2]. Calcium is a component of cell walls and regulates their construction; magnesium is the central atom in chlorophyll and a co-factor in producing ATP [1].
They also behave differently inside the plant [1]:
- Magnesium is mobile. Deficiency appears first on older leaves, as yellowing between the veins and at the leaf margins while the midrib stays green.
- Calcium is immobile. Deficiency appears on young leaves, which become distorted, sometimes abnormally dark green, with dry or brittle tips.
What the methods and sources recommend
| Source | Calcium | Magnesium | GH |
|---|---|---|---|
| PPS (water column) | 20–30 ppm | 2–5 ppm | not stated |
| EI (original article) | via a 4:1 Ca:Mg mix | via a 4:1 Ca:Mg mix | about 3 degrees or more; 3–5 for high light |
| Aquasabi wiki | via Ca:Mg ratio | above 10 mg/L recommended | not stated |
The magnesium figures disagree: PPS aims for 2–5 ppm while Aquasabi recommends more than 10 mg/L. Ratios vary too: Aquasabi reports 3:1 to 4:1 Ca:Mg as common practice and 2:1 as a newer approach, and proposes a Ca:Mg:K ratio of 2:1:0.5 [5]; EI’s author mixes the salts at 4:1 [7]. None of these sources presents its figure as the result of controlled experiments on aquatic plants, so treat them as working ranges.
EI’s author also argues that most aquatic plants do not need soft water, that a few soft-water species are affected by KH rather than GH, and that a GH of 5–10 with a KH of 5 suits nearly all plants [7]. That is his observation from growing many species, reported here as such.
Raising GH: the arithmetic
Epsom salt (MgSO4·7H2O) is 9.86 % magnesium. 1 g in 100 L adds 0.99 mg/L Mg, which is 0.99 ÷ 4.33 = 0.23 °dH. It raises GH without adding any calcium.
Gypsum (CaSO4·2H2O) is 23.3 % calcium. 1 g in 100 L adds 2.33 mg/L Ca, or 0.33 °dH, but it dissolves poorly: about 0.2 g per 100 mL [9], so it has to be stirred into a large volume of water or added dry. EI’s author notes the same difficulty and suggests calcium chloride as an alternative [7].
A commercial GH booster. Seachem Equilibrium declares 8.06 % calcium, 2.41 % magnesium and 23.0 % K2O, and its directions say 16 g per 80 L raises GH by 1 meq/L (3 dH) [6]. Working from the label percentages, 16 g in 80 L adds 16.1 mg/L Ca and 4.8 mg/L Mg: 0.60 mmol/L, which is 1.2 meq/L or 3.4 °dH, slightly more than the rounded figure in the directions. It also adds a large amount of potassium, about 38 mg/L by the label’s K2O figure, which is worth counting if you also dose potassium salts.
The hardness converter converts between degrees, mg/L as CaCO3 and mmol/L, and the dosing calculator shows the GH added by any amount of Epsom salt or gypsum.
When to add it
Aquasabi notes that plants consume only small amounts of magnesium [5]. EI’s author suggests adding about one degree’s worth after each weekly water change [7]. Aquasabi notes that once the tank is at the desired ratio, only the new water at each change needs enriching [5]. If your tap water is already hard, it may need nothing at all; check your supplier’s water report for calcium and magnesium in mg/L.
Questions
Can I raise GH with Epsom salt alone?
Epsom salt adds only magnesium, so GH rises but calcium does not. 1 g of MgSO4·7H2O in 100 L raises GH by about 0.23 °dH. EI's author suggests a calcium salt and magnesium sulfate together, at about 4:1 [7].
Calculators for this
- Dry fertilizer dosingConvert grams of KNO3, KH2PO4, K2SO4, Epsom salt or a trace mix into mg/L for your tank, or work back from a target ppm. Every molar mass is shown.
- Hardness converterConvert GH and KH between German degrees, mg/L as CaCO3, mmol/L, meq/L, grains per gallon and French or English degrees, with each definition shown.
Related guides
- FertilizersEstimative Index (EI) dosing: what the method prescribesEI as Tom Barr wrote it up: dose a surplus three times a week, change 50 % of the water weekly, and why that caps build-up at twice the weekly dose.
- FertilizersHow ppm from dry fertilizer salts is calculatedThe molar-mass arithmetic behind every dosing chart: grams of salt to mg/L of nitrate, phosphate or potassium, with worked examples and the common traps.
- FertilizersPPS-Pro: the lean daily dosing method and its targetsPPS-Pro as its author published it: two 500 mL bottles, 1 mL per 40 L a day, low water-column targets, and how it differs from EI in intent and arithmetic.
- Algae & problemsNutrient deficiency symptoms in aquarium plants: a symptom keyRead deficiency symptoms by where they start: old leaves for mobile nutrients, new growth for immobile ones. A symptom table with extension and grower sources.
Sources
- Nutrient Management Module No. 9: Plant Nutrient Functions and Deficiency and Toxicity Symptoms (4449-9). Montana State University Extension. Calcium: cell walls, deficiency distorts young leaves; magnesium: center of chlorophyll, interveinal chlorosis of older leaves; Ca immobile, Mg mobile. Accessed October 12, 2026.
- Nutrient Management Module No. 2: Plant Nutrition and Soil Fertility (4449-2). Montana State University Extension. Ca and Mg are secondary macronutrients, absorbed as Ca2+ and Mg2+. Accessed October 12, 2026.
- Hardness of water: measurement units. Wikipedia. 1 °dH = 10 mg/L CaO = 17.848 ppm as CaCO3 = 0.1783 mmol/L. Accessed October 12, 2026.
- Abridged standard atomic weights. IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW). Ca 40.078, Mg 24.305. Accessed October 12, 2026.
- The calcium-magnesium ratio. Aquasabi (aquascaping wiki). Gives 4.3556 mg/L Mg or 7.144 mg/L Ca per 1 °dGH; Ca:Mg 3:1–4:1 or 2:1; Mg above 10 mg/L recommended; Ca:Mg:K 2:1:0.5. Accessed October 12, 2026.
- Equilibrium. Seachem Laboratories. Guaranteed analysis K2O 23.0 %, Ca 8.06 %, Mg 2.41 %, Fe 0.11 %, Mn 0.06 %; 16 g per 80 L raises GH by 1 meq/L (3 dH). Accessed October 12, 2026.
- The Estimative Index of Dosing, or No Need for Test Kits. Tom Barr, The Barr Report (2005). GH 3–5 degrees covers high-light needs; CaCl2 or CaSO4 with MgSO4 at 4:1; add about 1 degree after each weekly change; GH target about 3 degrees. Accessed October 12, 2026.
- Plant requirements. Edward, Perpetual Preservation System. Mg 2–5 ppm, Ca 20–30 ppm. Accessed October 12, 2026.
- Calcium sulfate dihydrate (CID 24928). PubChem, National Library of Medicine. Solubility 0.2 g/100 mL at 20 °C. Accessed October 12, 2026.