GH in this tank above was about 10-12, with KH values of around 8-11 due to the limestone used in the tank. We would classify such water parameters as moderately hard.
Despite its fancy name, it just measures the total amount of Ca/Mg ions in the water (and other divalent cations). Calcium is present in most tap water. However, magnesium is often over-looked. If your tank has high GH, it can be that all of it is made out of Calcium ions, and no magnesium is present - in this case, magnesium still has to be dosed. Such data can often be found in local water reports.
Generally, hard tap water comes from limestone(CaCO3) regions - so its much more common to see high GH values with corresponding high KH values. However, these values do not have to match. It is possible to have high GH, low KH water. [Lots of Ca/Mg but no Carbonates] It is also possible to have low GH, high KH water. [Low amounts of Ca/Mg in water, but plenty of carbonates]
Plants and livestock are generally less sensitive to GH than KH. Plants that require softwater to grow well require low KH values, but not necessarily low GH values. These plants are sensitive to alkalinity, to be accurate, rather than General hardness. GH is important when keeping shrimp and shellfish.
Softwater planted tanks typically have GH as low as 1-3. While very hardwater tanks can have GH values in the 20+ dGH range. Shrimps and snails appreciate having calcium in the water, having 3 - 5 GH is beneficial.
As calcium is present in most tap water, calcium deficiencies are very rare in planted aquariums. Almost all cases of tip stunting attributed to calcium deficiency are mis-diagnosed and are caused by other variables instead.
Having 15ppm of Calcium and 5ppm of Magnesium will work fine for most tanks. Some plants such as Downoi and Rotala sunset favour higher GH levels.
For detailed care guide to aquarium plants, click here.