What does GH in the aquarium mean in practice?
GH mainly measures the combined effect of calcium and magnesium, which are needed by organisms, but the range required depends on the species.
When does it matter?
Conductivity or TDS does not replace GH because it also includes sodium, potassium, chloride and other ions.
Which solutions are related to it?
A drop test is used, RO water is remineralized with a known preparation and sudden changes are avoided during water changes.