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:
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Epsom salt (MgSO₄): Raises GH only, not KH. Adds magnesium. Approximately 3.5 grams per 10 gallons raises GH by 1 °dH. (or ~0.93 g per 10 liters per 1 °dH)
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Calcium chloride (CaCl₂): Raises GH, adds calcium. Approximately 2.5 grams per 10 gallons raises GH by 1 °dH.
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GH booster (commercial): Balanced Mg + Ca blend. Follow manufacturer instructions — dosing varies by brand. Typically 1/4 tsp per 5 gallons raises GH by ~1–2 °dH.
Caution: Always add minerals to a bucket of tank water and dissolve fully before adding to the tank. Sudden hardness changes stress fish. Raise or lower GH by no more than 2 °dH per day. Test water before and after dosing with a liquid test kit for accuracy.