What is a good level of kH in a planted aquarium ?
CARBONATE HARDNESS
Carbonate hardness, is a measure of water hardness caused by the presence of carbonate (CO3) and bicarbonate (HCO3) anions.It is a measure of the water’s buffering capacity; the higher the KH, the higher the pH in absence of other chemicals in the water, and the more resistant the water is to downward fluctuations when an acid is added.
​Pure distilled water that has 0 KH, and 0 other amounts of acid/bases will have a pH of 7. As KH increases, the pH will increase as well. KH has no innate value to most plants (some plants can use carbonates as a Carbon source and will grow better in higher KH water when deprived of other carbon sources, but this is very energy intensive for the plant). It’s sole purpose if any, is as a buffer to prevent tank water from getting overly acidic. Bacteria consumes it in small amounts as part of the ammonia oxidation process.
Aquasoil tanks in softwater countries often run at 1dKH or less (as does my farm tanks shown above), as the peat content of aquasoils absorb most of the available carbonates in water. Tropical fish and shrimp from soft water environments thrive in such a setting.
Raising and Lowering KH levels in a tank
KH levels in most hobbyists tanks are determined by what is available in their tap water. As with most soluble minerals, it is easier to add them into the tank water than remove them. The most common way to have softer water is by process it through an RO (Reverse osmosis unit) – such units can be costly so folks should ask whether do they really require very soft water to run their tanks.
Aquasoils generally contain peat and will lower KH of the water – however, this buffering does not last forever (a few months if the water is moderately hard).
Water softeners used by the public generally do not produce lower KH water. Such ion exchange devices generally exchange Calcium ions for Sodium or Potassium – and have little impact on the carbonate hardness of the water.Astute hobbyists can deplete KH in the water column by dosing HCL (Hydrochloric acid). However, this needs to be controlled carefully – we would not recommend this for the inexperienced.
Raising KH levels in the tank is easy. In a planted tank this is best done by adding KH2CO3 or K2CO3 (Potassium bi-carbonate/carbonate). The potassium functions as fertilizer for plants as well.
3.5 grams of KHCO3 in 100 litres of water raises KH by 1 dKH2.5 grams of K2CO3 in 100 litres of water raises KH by 1 dKHAnother easy way to raise the KH levels in a tank is by adding pieces of limestone into the filter or tank environment. Coral chips in a bag can also work. We recommend limestone over coral chips as stone is more easy to handle / remove.
Raising KH levels in the tank is easy. In a planted tank this is best done by adding KH2CO3 or K2CO3 (Potassium bi-carbonate/carbonate). The potassium functions as fertilizer for plants as well. 3.5 grams of KHCO3 in 100 litres of water raises KH by 1 dKH 2.5 grams of K2CO3 in 100 litres of water raises KH by 1 dKH Another easy way to raise the KH levels in a tank is by adding pieces of limestone into the filter or tank environment. Coral chips in a bag can also work. 
Tanks with limestone (Seiryu rock is shown here) naturally have elevated KH levels. Depending on the quality/type of limestone used, KH in the tank can rise from 0 to 10 within a week.
Source 2hraquarist.com