Aquarium Water Hardness Calculator
Calculate how much mineral supplement to add to raise your aquarium GH (general hardness) to the target level for fish and plants.
Water hardness refers to the concentration of dissolved minerals, primarily magnesium and calcium, in aquarium water.
GH (General Hardness) measures magnesium and calcium ions. It affects fish osmoregulation, plant nutrient uptake, and invertebrate shell growth.
KH (Carbonate Hardness / Alkalinity) measures bicarbonate ions. It buffers pH and prevents dangerous pH swings.
Hardness units:
| Unit | Description |
|---|---|
| °dH (German degrees) | 1°dH = 17.86 mg/L (ppm) of CaCO₃ equivalent |
| ppm / mg/L | Parts per million, 1 ppm = 1 mg per liter |
| mEq/L | Milliequivalents per liter, used in some test kits |
1 °dH = 17.86 ppm
GH targets by species:
| Species | Target GH |
|---|---|
| Neon tetra, discus, cardinal tetra | 1–5 °dH (soft) |
| Most community fish (guppies, platies) | 5–12 °dH (medium) |
| African cichlids, livebearers | 12–20 °dH (hard) |
| Shrimp (neocaridina) | 6–10 °dH |
| Shrimp (caridina/crystal shrimp) | 2–6 °dH (soft) |
| Planted tanks (general) | 3–8 °dH |
Common hardness additives:
The dose falls straight out of the definition. One degree of hardness is 17.848 mg/L expressed as calcium carbonate, and calcium carbonate is 100.09 g/mol, so a single degree needs 0.1783 millimoles of calcium or magnesium per liter. Multiply that by whatever the salt weighs per mole and you have the answer.
| Additive | Molar mass | Per liter, per 1 °dH | Per US gallon, per 1 °dH |
|---|---|---|---|
| Epsom salt, MgSO₄·7H₂O | 246.47 g/mol | 0.044 g | 0.166 g |
| Calcium chloride, CaCl₂·2H₂O | 147.01 g/mol | 0.026 g | 0.099 g |
Epsom salt raises GH and adds magnesium, and does nothing for KH. It is the cheapest option and it is in every pharmacy, but on its own it tips the calcium-to-magnesium ratio the wrong way. Shrimp keepers notice this before fishkeepers do.
Calcium chloride raises GH and adds calcium. Buy the dihydrate; the anhydrous form pulls water out of the air and the weight in your spoon stops meaning anything after a week in a damp cupboard.
Commercial GH boosters are a balanced magnesium and calcium blend, usually around a 1:3 magnesium-to-calcium ratio, and for a shrimp tank started from reverse-osmosis water they are worth the extra money. Dosing varies enough between brands that the label wins over any table, including this one.
Caution: Dissolve minerals fully in a cup or bucket of tank water first. Crystals that land on a plant crown or a shrimp will burn. Sudden hardness changes stress fish, so move GH by no more than 2 °dH per day in either direction. To lower it, dilute with reverse-osmosis or distilled water; nothing you add will bring it down. Test before and after with a liquid kit. Test strips are poor at the low end of the GH scale, which is precisely where soft-water and caridina keepers are working.
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