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

CARBONATE HARDNESS

Carbonate hardness (KH) is a measure of water hardness caused by the presence of carbonate (CO3) and bicarbonate (HCO3) anions. One common source of carbonates is limestone (CaCO3); water that has flowed through limestone areas is high in both Calcium and Carbonates. Carbonate hardness is a measure of the water's buffering capacity; the higher the carbonate hardness, the more carbonate (CO3)/bicarbonate (HCO3) ions are present. The higher the Carbonate hardness, the higher the pH will be in the absence of other chemicals in the water, and the more resistant the water is to downward fluctuations when an acid is added. Carbonate hardness is commonly measured in degrees (dKH).

One dKH corresponds to the number of carbonate and bicarbonate ions found in a solution of approximately 17.8 ppm (parts per million) Calcium Carbonate (CaCO3). dKH does not measure the calcium levels, it only takes into account the carbonate/bicarbonate component.

Carbonate hardness (KH) is often confused with General hardness (GH). KH measures carbonate hardness (CO3 and HCO3 anions). Whereas GH measures divalent cations such as Calcium and Magnesium (Ca and Mg cations). You can have a high GH and low KH or vice versa; either reading can be raised without changing the other. However, in most tap water systems, GH and KH values tend to be affected by whether the source water has contacted limestone (CaCO3), which raises both values simultaneously. Therefore, it is accepted practice in the hobby that the term "hard water" refers to water that is high in both GH and KH.

To be precise, however, we should quote GH and KH values separately, as the two values measures different things. When it is mentioned in the hobby that an aquatic plant requires "Softwater" it generally means that the plant requires low KH, not low GH. Aquatic plants are much more sensitive to KH values rather than GH values in the aquarium hobby.

Picky Eriocaulons require soft, low KH water. However, you can still grow them in tanks with a high GH, as long as the KH is low. GH is around 5-7 dGH in this tank, while KH is below 1dKH.

Here are some examples of common compounds and whether they contribute to GH or KH.

Compound	Cation	Raises GH?	Anion	Raise  KH?
Limestone	Ca2+ (Calcium)	Yes	CO3 (Carbonate)	Yes
Baking Soda	Na+ (Sodium)	No	HCO3 (Bicarbonate)	Yes
Potassium Carbonate	K+ (Potassium)	No	CO3 (Carbonate)	Yes
Magnesium sulphate	Mg2+ (Magnesium)	Yes	SO4 (Sulphate)	No
Potassium nitrate	K+ (Potassium)	No	NO3 (Nitrate)	No

​Pure distilled water has 0 KH. If there are no other chemicals dissolved such water will have also have a pH of 7. As KH increases, the pH will increase as well. 

Does adding CO2 change the KH?

Addition (or removal) of CO2 to a solution does not change its alkalinity, since the net reaction produces the same number of equivalents of positively contributing species (H+) as negative contributing species (HCO3 and/or CO3). Adding CO2 to the solution lowers its pH, but does not affect alkalinity.

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 it is very energy intensive for the plant to extract carbon from carbonates rather than CO2). 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.

When it is mentioned in the hobby that an aquatic plant requires "Softwater" it generally means that the plant requires low KH, this would usually also result in a low pH. However, it is the Carbonate hardness that matters. Having a low pH, by for example, super saturating higher KH water with CO2 - does not allow one to grow species that require softwater. You need to actually have low KH values.

Eriocaulon quinquangulare and 'Blood vomit' are softwater plants that require very low KH ranges (< 3dKH) to grow well.

Planted tanks generally do better at lower KH ranges than higher. With regards to growing plants, between 1-2 dKH you can keep sensitive softwater species.  (Some Eriocaulons and Tonina species will not survive in higher KH tanks). Between 2-7 dKH you can keep 97% of all commercial aquatic plants in optimal condition. (Some Rotala and Ammania species may have an easier time in softer water). Between 6 -12+ dKH you can probably grow 95% of species well, but some will be sub-optimal. Above 18 dKH or so, more plant growth issues start arising - at this level, hardy plants such as Java fern, Anubias, Vals, certain Swords and Crypts will still grow well, but many other species will stunt.

Many more sensitive freshwater fish species may have the same preference for lower or higher KH ranges, although the majority of commercially bred common ornamental fish function well through a large range (i.e. 1 to 10dkh). If you are intending to breed specific species, checking up on their requirements before hand is important.

Your tank needs to take into consideration both the requirements of both livestock and plants.

KH has a significant impact on livestock osmoregulation, and should not be changed rapidly for sensitive species such as ornamental dwarf shrimps. A 3dKH swing in KH value is significant enough to stress sensitive livestock. When purchasing such livestock, it makes sense to get them from a dealer with similar water parameters to your own.

If you need to prioritize one parameter to keep stable; keep the KH stable.

Managing water parameters well allows one to keep sensitive livestock in a planted tank.

Concerns about low KH and 'pH crash' affecting livestock / bacteria.

In well maintained planted tank setups - this almost never happens. Aquasoil tanks regularly have measurable KH levels of 1 dKH and below and thousands of tanks are run well this way without additional buffering. This mirrors the softwater states of many rivers/lakes - which often have pH ranges 6 and below. The accumulation of carbon dioxide over night and the subsequent depletion of CO2 during the light window causes pH in such natural lakes/river to vary more than 1 full point over a 6 hour window (dawn to noon).  There is no advantage or need to add buffers to have 2 or 3 dKH in a tank unless your specific livestock requires it.

Bacteria can and will populate low pH tanks. So low pH/KH environments are no bar to having a fully cycled tank contrary to old aquarium 'science'. For further information read: this link

Aquasoil tanks in softwater countries often run at 1dKH or less (as does the 2Hr Farm tank shown above), as the peat content of aquasoils absorb most of the available carbonates in water. Tropica fish/shrimp from softwater 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. These softeners lower GH (general hardness) rather than KH. This means that using such water softeners does not then allow one to grow/keep plants/livestock that require soft 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 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 (such as Seiryu rock) 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. 

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. 

For detailed care guide to the aquarium plants discussed above, click here.

For important water parameters for fishes, click here.

Conclusions

Carbonate hardness determines whether you can keep sensitive plants that require low KH. While it is easy to raise carbonate hardness by adding limestone to the tank/filter, it is difficult to reduce Carbonate hardness from tap water. For most folks, it makes more sense to keep aquatic plants/livestock that are suited to your tap water parameters. For folks that are determined to keep softwater plants but have hard tap water, RO systems are getting cheaper and easier to setup.