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.
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 require low KH to grow at all). 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 my farm tanks 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.
Astute hobbyists can deplete KH in the water column by dosing HCL (Hydrochloric acid). However, this needs to be controlled carefully - I 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 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. I 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.
For KH test kit and related products, click here for product reviews.