What is a good level for 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. A 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 buffering capacity of water; 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 will be to downward fluctuations when an acid is added. Carbonate hardness is usually 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 content, only the carbonate/bicarbonate component.

Carbonate hardness (KH) is often confused with total 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 high GH and low KH, or vice versa; you can raise one without changing the other. However, in most tap water systems, GH and KH tend to be influenced by whether the source water has been in contact with limestone (CaCO3), which raises both values simultaneously. Therefore, it is accepted 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 measure different things. When it is said in the hobby that an aquatic plant requires "soft water", this generally means that the plant requires low KH, not low GH. Aquatic plants are much more sensitive to KH levels than GH levels in the 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.

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

Does adding CO2 change the KH?

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

KH has no intrinsic 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 to extract carbon from carbonates rather than CO2). It's only purpose, if any, is as a buffer to prevent the tank water from becoming too acidic. Bacteria consume it in small amounts as part of the ammonia oxidation process.

When it is said in the hobby that an aquatic plant requires "soft water", this usually means that the plant requires a low KH, which would usually result in a low pH. However, it is the carbonate hardness that is important. If you have a low pH, for example by over-saturating higher KH water with CO2, you will not be able to grow species that require soft water. You really need low KH.

Eriocaulon quinquangulare and 'Blood vomit' are soft water plants that require very low KH ranges (less than 3dKH) to grow well.

Planted tanks generally perform better at lower KH ranges than at higher ones. In terms of growing plants, you can keep sensitive soft water species between 1-2 dKH. (Some Eriocaulons and Tonina species will not survive in higher KH tanks). Between 2-7 dKH you can keep 97% of all commercially available aquatic plants in optimum condition. (Some Rotala and Ammania species may do better 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, there are more problems with plant growth - 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 over a wide range (i.e. 1 to 10dkh). If you intend to breed certain species, it is important to research their requirements beforehand.

Your aquarium needs to take into account the needs of both animals and plants.

KH has a significant effect on osmoregulation in animals and should not be changed rapidly for sensitive species such as ornamental dwarf shrimp. A 3dKH change in KH is significant enough to stress sensitive animals. When purchasing such animals, it is advisable to buy them from a retailer with similar water parameters to your own.

If you have to prioritise one parameter to keep stable, keep KH stable.

Good management of water parameters will allow you to keep sensitive animals 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 well run this way without additional buffering. This reflects the soft water conditions of many rivers/lakes - which often have pH ranges of 6 and below. The accumulation of carbon dioxide overnight and the subsequent depletion of CO2 during the light window will cause the pH in such natural lakes/rivers to vary by more than 1 full point over a 6 hour window (dawn to midday). There is no benefit or need to add buffers to have 2 or 3 dKH in a tank unless your specific livestock require it.

Bacteria can and will colonise low pH tanks. So low pH/KH environments are not a barrier to having a fully cycled tank, contrary to old aquarium 'science'. 

For further information read: this link

Aquasoil tanks in soft water countries are often run at 1dKH or less (as is the 2Hr Farm tank shown above), as the peat content of Aquasoils absorbs most of the available carbonates in the water. Soft water Tropica fish/shrimp will thrive in such an environment.

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 to the tank water than to remove them. The most common way to get softer water is to run it through an RO (reverse osmosis) unit - such units can be expensive, so people should ask themselves if they really need very soft water to run their tanks.

Aquasoils usually contain peat and will lower the 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 exchangers generally exchange calcium ions for sodium or potassium - and have little effect on the carbonate hardness of the water. These softeners reduce GH (general hardness) rather than KH. This means that using such softeners will not allow you to grow/maintain plants/animals that require soft water.

Clever hobbyists can reduce the KH in the water column by dosing HCL (hydrochloric acid). However, this must be carefully controlled and is not recommended 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 simple way to increase KH levels in a tank is to add pieces of limestone (such as Seiryu rock) to the filter or tank environment. Coral chips in a bag can also work. We recommend limestone over coral chips as the stone is easier to handle/removed.

Tanks containing limestone (Seiryu rock is shown here) will naturally have elevated KH levels. Depending on the quality/type of limestone used, the 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 grow sensitive plants that require low KH. While it is easy to increase carbonate hardness by adding limestone to the tank/filter, it is difficult to reduce carbonate hardness from tap water. For most people it makes more sense to keep aquatic plants/animals that are adapted to your tap water parameters. For people who are determined to keep soft water plants but have hard tap water, RO systems are becoming cheaper and easier to set up.