GH/KH Hardness Test Kit Guide
What general hardness (GH) and carbonate hardness (KH) mean, how KH buffers pH, and how a drop-count titration kit measures both.
What it measures
A GH/KH test kit measures two related water parameters: general hardness (GH) and carbonate hardness (KH). GH reflects the amount of calcium and magnesium ions in the water, while KH measures the carbonates and bicarbonates. Both are reported in degrees (dGH, dKH) or parts per million (ppm).
What GH means
General hardness is the concentration of dissolved calcium and magnesium ions. It influences biological functions such as fish development and the moulting of shrimp. Soft-water species prefer low GH, while hard-water species such as livebearers and rift-lake cichlids prefer higher GH.
What KH means and why it buffers pH
Carbonate hardness measures the water's buffering capacity from carbonates and bicarbonates. Higher KH means more buffering capacity, so pH is harder to change and stays stable; low KH means little buffering, so pH can swing or crash. KH is therefore central to keeping pH steady.
Units
Hardness is expressed in degrees or ppm. One degree of general hardness (1 dGH) equals about 17.85 ppm as calcium carbonate, and one degree of KH (1 dKH) corresponds to about 17.9 ppm. Test kits usually let the user read either degrees or ppm from the same drop count.
Typical ranges
- General community freshwater: roughly 4-8 dGH and 4-8 dKH (about 70-140 ppm).
- Soft-water species: GH around 3 dGH or below and KH around 0-3 dKH.
- Hard-water species such as rift-lake cichlids: higher GH and KH, often above 10 dKH.
- KH is commonly kept at about 3 dKH or above to maintain stable pH.
How a titration kit works
GH and KH are measured by titration: reagent is added drop by drop to a fixed volume of tank water until the sample changes colour. Each drop represents one degree, so counting the drops to the colour change gives the hardness value. This drop-by-drop method reveals dosing needs and the characteristics of source water.
Using the kit
- Measure the exact sample volume the kit specifies into the test tube.
- Add the GH or KH reagent one drop at a time, swirling and counting after each drop.
- Stop at the colour change and record the number of drops as the hardness value.
- Repeat with the second reagent for the other parameter, and rinse the tube between tests.