Mixing example
Example: mix tap water at GH 10 / KH 5 with RO water at GH 0 / KH 0. A 50% blend produces about GH 5 / KH 2.5; a different pair of targets may require incompatible blend ratios.
How GH/KH mixing works
Each hardness value is converted to German degrees for calculation. GH and KH are treated as linearly mixable measured concentrations; the tool checks both independently and only gives a water recipe when the two results agree within 0.05 °dH.
Aquarium water mixing questions
What do GH and KH mean?
GH is general hardness, mainly dissolved calcium and magnesium. KH is carbonate hardness/alkalinity as reported by your test. They are separate measurements and are not interchangeable.
Does reverse-osmosis water always have zero hardness?
No. Enter the GH and KH you actually measured for both sources. The calculator does not silently assume either source is pure water.
Why can my GH and KH targets be incompatible?
One blend percentage must satisfy both values. If GH requires a different source ratio from KH, two-source mixing alone cannot reach both targets; the tool reports that instead of making up a recipe.
Which units can I convert?
German degrees (°dH), French degrees (°fH), mg/L as CaCO3 (ppm), and milliequivalents per litre (meq/L). The selected unit applies to every hardness input and result.
Is this a pH calculator or a fish-care recommendation?
No. pH does not mix linearly, and suitable aquarium water depends on species and other chemistry. This tool only estimates GH/KH blend proportions from supplied measurements.
Assumptions and limits
This is a mixing estimate, not a pH or species-care recommendation. It does not model remineralization, buffers, acids, substrate effects, evaporation or measurement error. Test your water and follow reliable aquarium-care guidance.