Turning RO or soft water into water with the GH and KH you want, using three cheap DIY salts: gypsum, Epsom salt and baking soda. Handy for shrimp, Rift Lake cichlids, or just matching a species. See also keeping fish in hard water.
Start from RO or your soft tap figures. A 3:1 to 4:1 calcium-to-magnesium ratio suits most fish and shrimp.
| Salt | Raises | Grams |
|---|---|---|
| Gypsum (CaSO₄·2H₂O) | GH, via calcium | – |
| Epsom salt (MgSO₄·7H₂O) | GH, via magnesium | – |
| Baking soda (NaHCO₃) | KH (and nudges pH up) | – |
Add gypsum and Epsom salt for GH, baking soda for KH. This calculator works out the grams of each to reach your target GH and KH at a chosen calcium-to-magnesium ratio.
GH is the calcium and magnesium fish and shrimp need; KH is the carbonate buffer that steadies pH. Gypsum and Epsom raise GH; baking soda raises KH and lifts pH.
No, salts only add hardness. To lower it, dilute with RO or rainwater using the RO mixing calculator.
More on matching fish to water in keeping fish in hard water and on what these numbers mean in water parameters.
Standard chemistry: one degree of hardness (dGH or dKH) equals 17.85 ppm as CaCO₃. Per litre and per degree that works out to about 30.7 mg gypsum and 44 mg Epsom salt for GH (split by your Ca:Mg ratio), and about 30 mg baking soda for KH. Gypsum raises GH with almost no effect on KH; baking soda raises KH and lifts pH. You cannot lower GH or KH with these salts, to soften water you dilute with RO. Add salts to the new water, stir until dissolved, and confirm with a test kit, since RO purity and salt hydration vary.