What is a good level of GH in a planted aquarium?

GH in this tank above was about 10-12 dGH, with KH values of around 8-11 dKH due to the limestone used in the tank. Limestone (CaCO3) dissolves slowly in water, adding both Calcium and Carbonates to the water. We would classify such water parameters as moderately hard.

General Hardness (GH)

Despite its fancy name, GH 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 added for plants.

GH is most common measured in degrees, with 1 dGH corresponding to the amount of Calcium present when 17.8ppm (Parts per million) of Calcium carbonate (CaCO3) is dissolved in 1 litre of water. This can be a confusing definition for aquarists as GH only counts the amounts of Calcium/Magnesium present, but using dGH as a unit means referring to calcium values in terms of units of Calcium carbonate dissolved per litre of water.

17.8ppm of Calcium carbonate dissolved in 1 litre of water (which is 1dGH) actually only gives 7.1ppm of Calcium (and 10.7ppm of Carbonates). Having 7.1ppm of Calcium in your water is equivalent to having 1dGH. Hobbyists often make the mistake of converting dGH to ppm in terms of Calcium by just multiplying the dGH value by 17.8 which is wrong, as this includes the mass of the Carbonate (CO3) anion. 

In short:

1 dGH = 7.1ppm of Calcium = 0.178 mmol/L of Calcium ions

1 dGH = 4.3ppm of Magnesium = 0.178 mmol/L of Magnesium ions

If you have for example, 14.2ppm of Calcium and 4.3ppm of Magnesium in your tap water, then you have exactly 3 dGH (2 dGH of Calcium + 1 dGH of Magnesium). In another example, if you have zero Calcium in your water, and 43ppm of Magnesium, you would still have water that measures 10 dGH. The dGH measurement/reading does not discriminate between Calcium and Magnesium (or other divalent ions). Calcium and Magnesium are both required by plants while some livestock such as shrimps care for Calcium but not Magnesium. Thus using the dGH measurement is really an imprecise tool for managing tank parameters compared to directly measuring Calcium and Magnesium levels. 

Seiryu rock is a popular rock used in aquascaping. However, Seiryu is a type of weathered limestone, and it will raise both the GH and KH. Tanks that make use of Seiryu rock should avoid plant/fish species that require very soft water.

Generally, hard tap water comes from limestone(CaCO3) regions - so its much more common to see high GH values with corresponding high KH values (which measure Carbonates present). 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).

Reading your local water report is an easy way to find out what is in your tap water. Follow this link on how to read your tap water report.

Plants and livestock are generally less sensitive to GH than KH. Plants that require soft water 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, or rather, specifically Calcium, is important when keeping shrimp and shellfish.

Soft water planted tanks typically have GH as low as 1-3 dGH, and most soft water plants grow fine at this range. While very hardwater tanks can have GH values in the 20+ dGH range. Shrimps and snails appreciate having calcium in the water, having 5 - 8 GH is a good range.

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. Some species such as Rotala florida, Cryptocoryne flamingo and Pogostemon helferi do appreciate higher Calcium levels. 5-7 dGH is a good level for these species. They grow faster and more robust with sufficient Calcium in the water.

Some sensitive species such as Rotala macrandra also do better when Magnesium levels are sufficient.

Having 15ppm of Calcium and  5ppm of Magnesium will work fine for most tanks. Having 30-50ppm of Calcium will be better for shrimps/shellfish and for plants that prefer higher Calcium levels. 

GH is often confused with KH, you read about KH head here.