What KH Is
KH, or carbonate hardness, measures the amount of bicarbonate (HCO3−) and carbonate (CO3²−) dissolved in your aquarium water. These ions act as a chemical buffer: they absorb acids as they build up, which keeps pH from swinging. Because of this, KH is often called alkalinity or buffering capacity. It is one of the main reasons a healthy tank holds a steady pH from week to week.
KH vs. GH vs. pH
These three parameters are related but not interchangeable. KH measures buffering capacity, GH measures dissolved calcium and magnesium, and pH measures how acidic or alkaline the water is right now. A tank can have high GH and low KH, or the reverse. KH controls how stable pH is, while pH only tells you where it currently sits.
Units: dKH vs. ppm
KH is reported in two common units. One degree of carbonate hardness (1 dKH) equals about 17.86 ppm (mg/L as CaCO3), so a reading of 6 dKH is roughly 107 ppm. Test kits differ in which unit they use, so always check the label before comparing numbers or following dosing advice.
Safe KH Levels
Freshwater Aquariums
Most community tanks do well at about 3 to 8 dKH (50 to 140 ppm). Soft-water species such as discus and many Amazon fish prefer about 1 to 4 dKH, planted CO2 tanks are often kept at 3 to 6 dKH, and African rift lake cichlids need about 10 to 18 dKH. Because KH controls how steady pH stays, consistency matters more than hitting an exact number.
Saltwater and Reef Aquariums
Natural seawater has an alkalinity of about 7 to 8 dKH, and most marine tanks target roughly 7 to 12 dKH. Reef tanks are commonly held near 7 to 9 dKH, since corals consume alkalinity as they build skeletons. In reefs, stability is critical because swings of more than about 1 dKH per day can stress corals.
Why KH Drops Over Time
KH is used up continuously. During the nitrogen cycle, bacteria oxidize ammonia and consume alkalinity in the process. CO2 injection, acidic substrates, driftwood, and peat add further acid load. In a heavily stocked tank with infrequent water changes, KH can fall toward zero, at which point pH can crash quickly and harm fish and filter bacteria.
Low KH and pH Crashes
When KH is very low, there is almost nothing left to neutralize incoming acids, so pH can drop sharply, sometimes overnight. Signs include fish gasping, lethargy, and a stalled nitrogen cycle, since nitrifying bacteria slow down in acidic water. Testing KH regularly is the best way to catch this early.
KH and CO2 in Planted Tanks
In tanks with injected CO2, pH, KH, and dissolved CO2 are linked, so pH and KH together can be used to estimate the CO2 concentration. Keeping KH steady makes pH shifts from CO2 predictable and helps avoid the dangerous levels that harm fish. Use a pH/KH/CO2 reference or calculator and confirm with a drop checker.
How to Test KH
Use a liquid titration test kit that counts drops of reagent until the sample changes color, then convert the result to dKH or ppm as the kit instructs. Test strips are less precise for KH. Test both your tank water and your tap water so you know what each water change adds.
How to Raise KH
The simplest method is sodium bicarbonate (baking soda): about 30 mg per liter raises KH by roughly 1 dKH. Dissolve it in a container of tank water first and add it slowly. For a gradual, passive increase, crushed coral or aragonite in the filter works well, but it also raises GH and pH, so it suits hard-water species rather than soft-water tanks. Commercial KH buffers are also available.
How to Lower KH
There is no practical chemical method for removing KH in a home aquarium. The reliable approach is dilution: use RO/DI or rainwater, or mix it with tap water, for water changes until KH reaches your target. Remove calcareous rocks or buffering media if they are driving KH upward, and make changes gradually.