KH (alkalinity) and baking soda
Key facts: KH, or total alkalinity, is the concentration of bases—mainly bicarbonate and carbonate—that buffer pond water against pH swings[1][2]; sodium bicarbonate (baking soda, NaHCO3) is a compound that supplies bicarbonate and can be used to raise alkalinity[3].
Total alkalinity is a measure of the concentration of bases—typically carbonate and bicarbonate—in water, reported in milligrams per liter as calcium carbonate (mg/L CaCO3)[1]. Alkalinity is the capacity of water to resist acidification and is distinct from pH, which is an absolute measurement on the pH scale[2]. In water with moderate to high alkalinity and similar hardness levels, pH tends to be neutral or slightly basic (7.0–8.3) and does not fluctuate widely[4].Keepers monitoring pH in a koi pond often find that alkalinity determines how stable that pH will remain from day to night, since dissolved carbon dioxide from respiration and photosynthesis constantly pushes pH up and down unless buffered[1][4].What KH measures
Total alkalinity is the stoichiometric sum of bases in solution, conventionally measured by titrating a water sample with a strong acid such as HCl until a defined endpoint (commonly pH 4.5) is reached[2]. In most surface waters, carbonate and bicarbonate make up the majority of total alkalinity due to dissolution of carbonate rocks and related weathering processes[2]. Results are usually reported in mg/L as CaCO3 or in parts per million, which are equivalent units[1].A desirable range for total alkalinity in fish culture water is given as 50–150 mg/L as CaCO3, with an acceptable range of more than 20 mg/L and less than 400 mg/L for ponds[1]. A separate source on pond interactions states that total alkalinity for fish culture generally falls between 75 and 200 mg/L CaCO3[4]. Total alkalinity below 20 mg/L limits primary productivity in water[1].Buffering and pH crash
Increasing total alkalinity helps buffer water against pH changes[1]. During the day, algae and plants remove carbon dioxide for photosynthesis and pH rises; at night, pH falls as carbon dioxide accumulates from respiration of fish, plants, and other organisms[1][4]. In water with low total alkalinity, these swings are larger because there is less base available to neutralize the acid produced by CO2[4].The relationship among alkalinity, pH, and CO2 can be estimated: a factor corresponding to measured pH and water temperature is multiplied by the measured alkalinity (mg/L as CaCO3) to approximate CO2 concentration[4]. Most fish species do well within a pH range of 6.5 to 9.0, and chronic pH below 6.5 may reduce reproduction and is associated with fish die-offs, sometimes occurring in late winter[1]. A pH reading below 4.5 indicates strong mineral acidity, which is typically harmful to fish and expensive to neutralize[1]. A collapse of buffering capacity that lets pH fall rapidly—sometimes described informally as a pH crash—is consistent with these low-alkalinity, low-buffering conditions described in the aquaculture literature[1][4].Alkalinity also affects ammonia toxicity indirectly: since alkalinity tends to raise and stabilize pH, and ammonia becomes more toxic as pH increases (favoring the toxic NH3 form over the less toxic NH4+ form), ammonia will be more toxic in waters with high total alkalinity[4]. See ammonia and the nitrogen cycle for related background.Using baking soda (sodium bicarbonate)
Sodium bicarbonate (NaHCO3), commonly known as baking soda, is a salt composed of a sodium cation and a bicarbonate anion[3]. Because bicarbonate is a principal component of total alkalinity, sodium bicarbonate is a direct source of alkalinity in water. High bicarbonate alkalinity in soft water can be produced by sodium and potassium carbonates, which are more soluble than the calcium and magnesium carbonates that cause hardness[4].In pond management generally, low-alkalinity waters benefit from the addition of lime, and crushed agricultural limestone can be added to increase both the total alkalinity and hardness of water with low pH[1]. Agricultural limestone is also described as a way to increase calcium concentrations and carbonate-bicarbonate alkalinity together[4]. The cited sources do not provide koi-specific dosing instructions for sodium bicarbonate in ponds, so any application should be based on measured alkalinity and gradual, monitored adjustment rather than a fixed recipe.GH (hardness)
General hardness (GH) refers to the total hardness of water, primarily determined by calcium and magnesium concentrations[1]. Hardness is traditionally measured by chemical titration and reported in milligrams per liter as calcium carbonate (mg/L CaCO3)[4]. Calcium carbonate hardness is a general term indicating the total quantity of divalent salts present; it does not specifically identify whether calcium, magnesium, or another divalent salt is responsible[4].Hardness is commonly confused with alkalinity, since both are reported in the same units (mg/L CaCO3)[4]. However, they are not interchangeable: where limestone is responsible for both hardness and alkalinity, the concentrations will be similar, but where sodium bicarbonate is responsible for alkalinity, it is possible to have low hardness and high alkalinity[4]. Calcium and magnesium concentrations cannot simply be summed to obtain total hardness, since they are each reported as mg/L rather than mg/L as CaCO3[1].Environmental calcium is required for fish to re-absorb salts; in low-calcium water, fish can lose substantial sodium and potassium into the water and must expend energy reabsorbing them[4]. For general pond water quality, a calcium hardness of 100 to 250 mg/L CaCO3 has been cited as desirable alongside moderate to high total alkalinity (75–200 mg/L, not less than 20 mg/L)[4]. Stabilizing pH is helped by keeping calcium hardness close in value to total alkalinity[1].Testing and interpreting reports
There is no single standard list of parameters that water-testing laboratories measure, and units of measure vary among labs; results are often given in mg/L or ppm, which are equivalent[1]. An example laboratory report cited in aquaculture literature listed hardness at 264 ppm CaCO3 and alkalinity at 301 ppm CaCO3 from the same sample, illustrating that the two values are measured separately even though expressed in the same units[1]. Keepers using test kits and monitors at pond-side should be aware that a prompt pH measurement, taken within about 30 minutes of sampling, is needed to minimize error, since pH readings can shift during shipment of a sample to a lab or during storage, as organic matter and dissolved CO2 continue to change[4][1]. Total alkalinity and total hardness, by contrast, do not change significantly over a two- to three-day shipping period[1].Common questions
What is a good KH range for a koi pond?
Why does pH crash when KH is low?
Does baking soda raise both KH and GH?
Are KH and GH the same measurement?
How does alkalinity affect ammonia toxicity?
Because high total alkalinity tends to raise and stabilize pH, and ammonia becomes more toxic at higher pH, ammonia is more toxic in waters with high total alkalinity[4].
Related koi topics
References
- ^ SRAC Publication No. 4606: Interpretation of Water Analysis Reports for Fish Culture. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4606%20Interpretation%20of%20Wa…. Retrieved November 2026.
- ^ Alkalinity (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Alkalinity. Retrieved November 2026.
- ^ Sodium bicarbonate (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Sodium_bicarbonate. Retrieved November 2026.
- ^ SRAC Publication No. 464: Interactions of pH, Carbon Dioxide, Alkalinity and Hardness in Fish Ponds. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/464%20Interactions%20of%20pH%2C…. Retrieved November 2026.