Water chemistry2026-10-04

What should pH and KH be in a koi pond?

Keep a koi pond's pH steady somewhere between about 7.0 and 8.5, and keep its KH (carbonate hardness, or total alkalinity) between 100 and 250 mg/L as calcium carbonate, which is the range veterinary sources give for koi. KH is the buffer that keeps the pH from swinging with the algae every day and from crashing when the filter bacteria use it up. Plain baking soda raises KH by about 10 mg/L for every 2.24 ounces added per 1,000 gallons, and a sodium bicarbonate maker says not to raise alkalinity more than 50 mg/L in a day.

What the authoritative sources say

For pH, the Southern Regional Aquaculture Center's SRAC 464 (Wurts and Durborow) says "A desirable range for pond water pH would be close to that of fish blood (i.e., 7.0 to 8.0)." Dr. N. Saint-Erne, a certified aquatic veterinarian, wrote in Water Quality in Koi Practice (WSAVA 2019) that "Most freshwater fish are highly adaptable to slow changes in the pH as long as it is not too extreme (less than 5.5 or above 8.5)," and "it is very important that the pond water has a stable pH." The Merck Veterinary Manual puts the lethal limits "for many species" at "approximately 4 and 10."

For KH, Saint-Erne says "Ideal alkalinity for koi is in the 100-250 mg/L range" and "As the alkalinity falls, the water in a pond may experience sudden, and deadly, pH shifts." Merck agrees on 100 to 250 mg/L for most fish and says "When TA is < 50 mg/L, water is considered low in alkalinity, and buffering ability will not be adequate to prevent major pH fluctuations." Dr. Gregory Lewbart of the NC State veterinary college (Koi Medicine) says that below 50 ppm you should "add crushed coral or dolomite to the system to increase buffering capacity."

Target ranges by source. Alkalinity and KH are both reported in mg/L (ppm) as calcium carbonate.
Source	pH	KH or alkalinity
SRAC 464 (fish ponds)	7.0 to 8.0 desirable	75 to 200 mg/L desirable for fish culture
Saint-Erne, aquatic veterinarian (koi)	Avoid below 5.5 or above 8.5	100 to 250 mg/L
Merck Veterinary Manual (freshwater fish)	Lethal near 4 and 10	100 to 250 mg/L; under 50 is low
Lewbart, NC State (koi)	Not given	Add buffer below 50 ppm
The Pond Guy (retailer)	6.5 to 8.5	50 to 200 ppm

The daily swing and the crash

SRAC 464 says "Pond pH varies throughout the day due to respiration and photosynthesis": algae and plants take carbon dioxide out of the water in daylight, so pH rises, and everything in the pond breathes it back out at night, so pH falls. Claude Boyd of Auburn University gives the size of the swing in fish ponds, "rising from 6 to 7 in the early morning to 9 to 10 in the afternoon in ponds with low alkalinity (0-40 mg/L), and from 7 to 8 in the morning to 8 to 9.5 in the afternoon in ponds that have naturally higher alkalinity or have been limed." Bicarbonate soaks up the acid from that nighttime carbon dioxide, which is why KH narrows the swing.

A koi pond also spends its KH steadily. Merck says "biofilter bacteria acidify the water via the nitrification process," and Boyd puts a number on it: "Nitrification of 1 mg/L of ammonia nitrogen can reduce alkalinity by 7.14 mg/L," and in "plastic-lined intensive ponds, nitrification can cause drastic losses of alkalinity in a matter of days." A lined koi pond has no soil or limestone to replace what is lost. Once the bicarbonate is gone, nothing neutralizes the acid any more and the pH can fall several units overnight. That is the pH crash.

pH and ammonia

SRAC 464 says "Ammonia becomes more toxic as pH increases." The share of a total ammonia reading that is in the toxic free form at 75°F rises about tenfold for each pH unit, so the afternoon peak of the daily swing is when ammonia does the most harm. Merck's harm line is 0.05 mg/L of free ammonia. New pond syndrome and ammonia covers the water change arithmetic once the reading is high.

Free ammonia from a 1 mg/L total ammonia reading at 75°F, from the Emerson equation that the PondTools ammonia calculator uses.
pH	Free ammonia	Compared with Merck's 0.05 mg/L line
7.0	0.005 mg/L	About a tenth of it
7.5	0.016 mg/L	About a third of it
8.0	0.050 mg/L	At the line
8.5	0.143 mg/L	About three times over
9.0	0.346 mg/L	About seven times over

What dealers, clubs and owners do

The Pond Guy says "The ideal pH level for koi ponds is between 6.5 and 8.5," and advises owners to "Make adjustments in 0.5 pH increments to avoid shocking your fish." Its carbonate hardness page puts KH "between 50 ppm and 200 ppm," and its page on lowering pH suggests a partial water change and its own pH products. Aquascape's Alkalinity Booster page (read through its Canadian distributor's copy, because Aquascape's site refused the fetch) says "We recommend building the alkalinity/KH slowly as a rapid increase in pH may cause stress to pond fish."

The Koi Club of San Diego recommends "buffering your KH to a level of at least 150 PPM," notes that "Some Pond hobbyists prefer to keep their KH level over 200 PPM," and says "We recommend adding 1 cup of baking soda per 1,000 gallons of pond water daily until your KH level has increased to 150 PPM," with a test "at least once per week and especially after rain." Next Day Koi gives a smaller dose; its page refused our fetch, and the search snippet reads "add ¼-½ cups of baking soda per 1000 gallons of water in your pond."

On Garden Pond Forum in 2013, ShawnC reported an 1,800 gallon pond with six large koi after "a huge raid storm 2 days ago that dumped 5 inches into my pond." He had read "amonia was at 4 and ph was at 9," then the next morning "amonia of 2 and PH of below 5," and lost all but one fish despite "2 cups of baking soda." crsublette replied that "rain eats KH as well and KH creates a stable pH," said that raising KH from zero to five test drops in that pond would take "1.8 pounds (or 30 ounces) of baking soda," and told him to buy a KH test kit.

Where the tiers differ

The Pond Guy's 50 ppm floor sits right at the edge of what Merck calls low, while the veterinary sources start at 100 mg/L and the San Diego club at 150 ppm. A lined koi pond with a working biofilter is better served by the higher floor, because nitrification keeps drawing KH down between tests.

Measuring by the cup hides how much you are adding. USDA FoodData Central lists a teaspoon of baking soda at 4.6 grams, which makes a 48 teaspoon cup about 221 grams, and King Arthur lists half a teaspoon at 3 grams, which makes a cup about 288 grams. At the 1.68 ratio below, one cup in 1,000 gallons raises alkalinity by about 35 to 45 mg/L, inside the 50 mg/L daily limit, so the San Diego dose holds up. Next Day Koi's quarter to half cup comes to roughly 9 to 23 mg/L.

Baking soda also does not hold an algae-driven pH down. Craig Tucker of Mississippi State added sodium bicarbonate to pond water at pH 10.0, and even 100 mg/L of it in water with 50 mg/L alkalinity only brought the pH to 9.6; "results are not spectacular," he wrote. KH prevents the nighttime crash, and the afternoon peak comes down only when there is less algae.

How to raise KH with baking soda

Boyd says "About 1.68 mg/L of sodium bicarbonate are necessary to restore 1 mg/L of alkalinity." For a 10 mg/L rise that is 16.8 mg/L of baking soda. One thousand gallons is 3,785 liters, so 16.8 times 3,785 is about 63,600 milligrams, or 63.6 grams, which is 2.24 ounces. Arm & Hammer's pool guidance agrees closely: "1.5 lbs. of baking soda per 10,000 gallons of water will raise alkalinity by about 10 ppm," which is 2.4 ounces per 1,000 gallons. Pentair Aquatic Eco-Systems' sodium bicarbonate instructions say "Do not adjust alkalinity more than 50 mg/l every 24 hours" and "Do not adjust the pH more than one unit every 24 hours."

Plain baking soda (sodium bicarbonate) at 1.68 mg/L per mg/L of alkalinity. The 50 mg/L column is the most Pentair says to add in one day.
Pond volume	To raise KH 10 mg/L	To raise KH 50 mg/L
1,000 gallons	2.24 oz (63.6 g)	0.70 lb (318 g)
3,000 gallons	6.73 oz (191 g)	2.10 lb (954 g)
5,000 gallons	11.2 oz (318 g)	3.51 lb (1,590 g)
10,000 gallons	1.40 lb (636 g)	7.01 lb (3,180 g)
20,000 gallons	2.80 lb (1,272 g)	14.0 lb (6,360 g)

Take a 5,000 gallon pond testing 40 mg/L that you want at 120 mg/L. The rise is 80 mg/L, so you need 80 times 1.68, or 134.4 mg/L of baking soda. Five thousand gallons is 18,927 liters, and 134.4 times 18,927 is about 2.54 kilograms, or 5.6 pounds. Split it so no day adds more than 50 mg/L: 3.5 pounds on the first day, then retest, then 2.1 pounds on the second.

What to do

Test KH and pH together. Test pH at dawn and again in late afternoon, so you see the low and the high.
If KH is under 100 mg/L, raise it with plain baking soda (not baking powder) weighed from the table, dissolved in a bucket of pond water and poured where the water moves. Retest after 24 hours before the next dose.
Retest KH weekly in summer, when feeding and nitrification are highest, and after any heavy rain.
If the dawn pH is in range and only the afternoon reading is high, the cause is photosynthesis. Cut feeding, add shade and plants that compete with algae, and check the ammonia at that afternoon pH.
If the dawn reading is high too, lower it gradually. Lewbart's advice is "Gradual water changes to bring pH back into normal range," using source water you have tested, no more than The Pond Guy's half unit at a time.

Do not pour acid into a pond with low KH. The acid uses up what buffer is left, and then the pH drops all at once, which is the crash you were trying to avoid.

The PondTools calculators for this

Every dose here depends on the real volume. The PondTools volume calculator (also as a standalone page) takes length, width, average depth and shape and returns gallons, and a pond traced in the PondTools app gets its volume from the outline. The PondTools ammonia calculator (standalone) takes the total ammonia reading, whether your kit reports it as ammonia or as nitrogen, the pH and the water temperature, and returns the free ammonia; run it at both your dawn and afternoon pH. The PondTools water change calculator (standalone) takes the volume, the current and target readings, what the replacement water reads and the percent replaced each time, and tells you how much water to change to bring ammonia down. For alkalinity and liming in a farm pond, see reading a pond water test.