Two readings that explain most of the trouble people blame on pH. General hardness (GH) is the calcium and magnesium content — it decides which species will actually thrive, since soft-water tetras and hard-water livebearers want opposite ends of it. Carbonate hardness (KH) is the water's buffering capacity: when KH is low, pH swings sharply between water changes no matter how carefully you dose adjusters.
Both are read by counting drops rather than matching a colour block, so there is no scale to run off the end of. It is a freshwater kit.
What is in the kit
- 1 GH test solution and 1 KH test solution
- 2 capped glass test tubes
- Colour card and instructions
The order to test in
If pH will not hold, test KH first. Raising KH stabilises pH; chasing pH directly while KH is low just moves it back and forth.
Looking after the kit
Rinse each tube with tap water straight after use — dried reagent stains the tube and throws out the next reading. Never wash tubes with detergent; the residue is harmful if it goes back into the tank. Shake the reagent bottles firmly, but invert the capped tube gently.
Carbonate hardness on its own: KH Hardness Mini Test Kit.
API General & Carbonate Hardness Test Kit on the API website
