Water quality2026-10-03

New pond syndrome: how much water to change for ammonia

A new koi pond takes six to eight weeks or more to grow the bacteria that clear ammonia, so ammonia and then nitrite climb after the first fish go in. How much of the reading is the free (un-ionized) form that damages gills depends on pH and temperature. If your fresh water tests zero, the share to replace is one minus the target divided by the current reading, so taking a 3,000 gallon pond from 2 mg/L down to 1 mg/L means a 50 percent change, or 1,500 gallons. In the meantime, stop feeding and hold off on adding fish.

What the authoritative sources say

The Merck Veterinary Manual says "Most water quality test kits measure Total Ammonia Nitrogen (TAN) which includes both toxic (unionized) ammonia (NH3) and less toxic ammonium (NH4+)," and "The percentage of Total Ammonia Nitrogen that is present in the toxic, unionized form (NH3) increases as pH and temperature increase. Of these, pH has more influence than temperature." The line it draws for harm applies to the free form: "A calculated NH3 concentration of 0.05 mg/L is high enough to cause histologic changes to gill tissue and is considered harmful to fish."

The University of Florida fact sheet Ammonia in Aquatic Systems gives the free share at 75.2°F as 0.0052 of the reading at pH 7.0, 0.0502 at pH 8.0 and 0.3456 at pH 9.0, and SRAC Publication 463 says "For every pH increase of one unit, the amount of toxic un-ionized ammonia increases about 10 times." So at 75°F a 1 mg/L reading is a tenth of Merck's line at pH 7.0, right on it at pH 8.0, and about seven times over it at pH 9.0. Check your kit's units as well: Hach's Method 8155 reports ammonia as nitrogen and says "To change the results to mg/L ammonia (NH3), multiply the result by 1.22."

On timing, Merck's aquarium page says "It can take up to 8 weeks for a biofilter in a tropical fish tank to become established." Its environmental page says ammonia rises "during the first 2-3 weeks," nitrite follows, "It usually takes ~6 weeks for a new biofilter to become completely established," and the filter has matured "when TAN and NO2 have peaked and dropped, followed by a rise in NO3." Florida says "A new biofilter requires six to eight weeks," and Dr. Jessie Sanders, a board-certified fish veterinarian in California, says "4-6 weeks." These describe tanks at aquarium temperatures; SRAC 463 notes that "lower water temperatures slow down aerobic bacterial activity, thus slowing the nitrification process."

On treatment, Merck says "a water change (≥50%) should be done as soon as high NH3 levels are detected" and "Feeding should be discontinued or substantially reduced until the problem has been corrected," and warns that source water can add ammonia "(ammonia released from chloramines during dechlorination)." Florida says "A 25% to 50% water change will help remove some ammonia, assuming the incoming water source does not contain ammonia." Sanders sets a ceiling: "Remove no more than 50% of the total volume of the tank or pond." Oklahoma State's NREM-100 says "The least expensive and most reliable method of avoiding water quality problems is to stock small numbers of fish and limit or totally skip the use of fish feed."

Salt is for the nitrite stage. Merck says chloride "serves as an antagonist for NO2 at the gill epithelium," that "Many ornamental ponds and aquaria are maintained with residual chloride levels because of the addition of salt (1-3 ppt)," and that in garden ponds "salt may still be used to halt mortality during a sudden increase in NO2 exposure." No source here gives a salt dose for ammonia, and Florida says salinity lowers the free share only "slightly."

What dealers, contractors and owners do

Kloubec Koi Farm (Amana, Iowa) says "Generally, 10-20% of the pond water should be drained weekly," and for dangerous ammonia or nitrite, "Perform an immediate water change of at least 1/3 to 1/2 of your total pond volume," then "Follow up with 20-30% daily changes if water quality remains poor." Dreamscapes Water Gardens (Lebanon, Pennsylvania) says "Depending on the severity of the ammonia, a 25-50 percent water change should be performed immediately," and recommends pond salt for the slime coat. Hydrosphere Water Gardens (Bradford, Ontario) says "anything above 1.0 ppm should be acted upon immediately" with "20% to 30% water changes daily." The longest cycling times come from a British dealer: Pondkeeper says "A new pond will take up to 80 days to cycle" and "it may take up to 3 months for the pond to mature."

On Garden Pond Forum, eirrac11 (2011) stocked a new 1,200 gallon pond in Arkansas within a month and lost most of the fish; the pet store said to "immediately drain at least 1/2 of the pond, more if possible." fishin4cars disagreed, "if you do to large of a water change it will try and cycle again," and recommended "about a 30% water change wait for a few days then do another 25%-30% water change." In 2023 dansage had a newly built 8,500 liter pond with 18 fish and nitrite at "2.0mg/ltr" after 3.5 weeks. WaterGardener told him "Your 10% water changes are fine," warned that "Changing a large percentage of the water and trying to keep the nitrite at zero while the pond is cycling will only delay the process," and suggested Seachem Prime, whose protection "will last only 48 hours."

Where the tiers differ

No source in either tier works the volume out from the reading. Merck says at least 50 percent, Sanders says no more than 50, Florida says 25 to 50, and the dealers and forum posters range from 10 to 50, whatever the reading. A fixed percentage cuts every reading by the same fraction, so a 30 percent change takes 8 mg/L to 5.6 mg/L, still more than five times Merck's line at pH 8.0 and 75°F.

Nothing in the authoritative sources supports the fear that a big change restarts the cycle. Florida says nitrifying bacteria live "on the surface layers of the mud/substrate and on plants or other structures," so they stay when water leaves; untreated tap water is what harms them, and Kloubec says "Chlorine is highly toxic to fish and beneficial bacteria." WaterGardener's point is a fair one: the bacteria feed on ammonia and nitrite, so a pond held at zero cycles more slowly.

The alarm levels that circulate (0.25 ppm, 1.0 ppm) are total readings with no pH attached. Merck's 1 to 3 g/L "strongly recommended during the recovery and healing periods" sits in its surgery section, so it isn't a dose meant for a cycling pond. The 80 day figure cites no study, though the cold-water slowdown SRAC 463 describes makes it plausible for a pond filled in early spring.

The water change arithmetic

If the replacement water reads zero, the share to replace is one minus the target divided by the current reading. Set the target from Merck's 0.05 mg/L free ammonia line, so it moves with pH: at 75°F about 5 percent of the reading is free at pH 8.0, putting the target near 1.0 mg/L, and about 14 percent is free at pH 8.5, putting it near 0.35 mg/L.

A 3,000 gallon pond at 75°F, replacement water reading zero. Free ammonia comes from the Emerson equation that the PondTools ammonia calculator uses. The counts of 25 percent changes assume the fish add no ammonia in between, so expect to need more.
Total ammonia reading	Free ammonia at pH 8.0	One change to reach 1.0 mg/L	25 percent changes to 1.0 mg/L	Free ammonia at pH 8.5	One change to reach 0.35 mg/L	25 percent changes to 0.35 mg/L
1 mg/L	0.05 mg/L	None	None	0.14 mg/L	65 percent, 1,950 gal	4
2 mg/L	0.10 mg/L	50 percent, 1,500 gal	3	0.28 mg/L	83 percent, 2,475 gal	7
4 mg/L	0.20 mg/L	75 percent, 2,250 gal	5	0.57 mg/L	91 percent, 2,738 gal	9
8 mg/L	0.40 mg/L	88 percent, 2,625 gal	8	1.14 mg/L	96 percent, 2,869 gal	11

Anything over 50 percent means splitting the job: change half, retest the next day, change again. If your tap water reads 0.25 mg/L after dechlorinating, taking 2 mg/L to 1.0 mg/L needs 57 percent instead of 50, and no number of changes gets below 0.25. The fractions work for a pond of any size, but half of a 20,000 gallon pond is 10,000 gallons of dechlorinated water, and Florida limits this method to "tanks or very small ponds."

What to do

Test total ammonia, nitrite, pH and temperature together, and test your tap water for ammonia after dechlorinating it.
Stop feeding and add no fish. Light stocking is Oklahoma State's cheapest fix, and how many koi per gallon covers the numbers.
Work out the free ammonia. If it is over 0.05 mg/L, work out the change that reaches your target and do it in steps of no more than half the pond, with dechlorinated water matched to pond temperature.
Retest 24 hours later and repeat. Keep the aeration running; SRAC 463 says high oxygen "will slightly reduce the toxic effect."
If nitrite climbs, add salt, which Merck says can halt deaths during a nitrite spike, within the 1 to 3 ppt range Merck says many ornamental ponds are maintained at. One ppt is about 3.8 grams per gallon, or about 25 pounds on 3,000 gallons; salt for a koi pond has the details. After each water change, replace only the share of salt that left.
Add fish only after ammonia and nitrite have both peaked and fallen and nitrate is rising.

Do not try to fix the reading by moving the pH. Merck warns that when total ammonia is over 5 mg/L at a pH below 7, the fish should be moved, because a water change raises the pH and turns more of the ammonia toxic.

The PondTools ammonia and water change calculators

The PondTools ammonia calculator (also as a standalone page) takes the total ammonia reading, whether your kit reports ammonia or ammonia as nitrogen, the pH and the water temperature in degrees Fahrenheit, and returns the free ammonia. The PondTools water change calculator (also as a standalone page) takes the pond volume, the current reading, the target reading, what the replacement water reads and the percent replaced each time, and returns the single change and the number of smaller changes. A pond traced in the PondTools app fills in the volume.