Koi pond care
How much salt to add to a koi pond
For the 0.1 percent maintenance level, which is 1 part per thousand or 1 gram per liter, add 0.83 pounds of plain sodium chloride per 100 US gallons. For the 0.3 percent treatment level add 2.5 pounds per 100 gallons, which is 3 grams per liter. On a 3,000 gallon pond that is 25 pounds and 75 pounds, and on 10,000 gallons it is 83 pounds and 250 pounds. Add it in thirds over three days, check the level with a salt meter instead of trusting cup measures, take the plants out at 0.3 percent, and plan the water changes, because they are the only way the salt leaves.
What the authoritative sources say
The unit that ties everything together comes from the Merck Veterinary Manual: calculate "the total volume in liters (3.8 L = 1 gal.), remembering that 1 g/L = 1 part per thousand (ppt)." Since 1 gram per liter is 0.1 percent by weight and 100 US gallons weighs about 834 pounds, 0.1 percent of it is 0.83 pounds. The Koi Club of San Diego (Carolyn Weise) says "The recommended dosage for salt is 0.3%," that "At 0.3% this would be 2.5 lbs. of non-iodized salt per hundred gallons of water," added "in small, dissolved increments until reaching the 0.3% level, over a 3 day period," and, in capitals, "NEVER USE IODIZED SALT."
Merck's Management of Aquarium Fish page sets the levels by purpose: "Most freshwater fish should be able to tolerate a salinity of 3 g/L on an almost indefinite basis"; "Permanently increasing salinity to 2 to 3 g/L in a freshwater system can minimize some parasitic protistans"; and, for fish recovering from surgery, 1 to 3 g/L "is strongly recommended during the recovery and healing periods." On nitrite, Merck says salt "may still be used to halt mortality during a sudden increase in NO2 exposure." That is the one place salt belongs in a new pond fighting ammonia and nitrite: it blocks nitrite at the gill while the filter catches up, and it does nothing for the ammonia itself.
The cautions come from the club. "Many plants in the pond will also suffer and die at this salt level," and where they cannot be removed "it is recommended the salt level not exceed 0.1%." For external parasites "2-3 days would be sufficient to kill most populations," "To use salt all year will serve to promote salt resistant parasites," and salted water "can't be discharged onto the garden." The Pond Guy adds: "Salt will not evaporate or get filtered out, so the only time you need to add more salt is when you change out your water." Its doses are in cups, "2½ cups of salt per 100 gallons of water" without plants and "1¼ cups" with them. A cup measures volume, so what it weighs depends on the salt. USDA FoodData Central (as tabulated by CupOrGram) puts fine table salt at "1 cup = 292g," and coarse crystals pack lighter. At that weight 2½ cups is about 1.6 pounds, which brings 100 gallons to roughly 1.9 ppt, and 1¼ cups lands on 1 ppt. No source fetched for this article names a fish species that fails at 0.3 percent.
| Percent | ppt and g/L | lb per 100 gal | Cups (fine salt) per 100 gal | 1,000 gal | 3,000 gal | 10,000 gal | 20,000 gal |
|---|---|---|---|---|---|---|---|
| 0.1 percent (maintenance, nitrite protection) | 1 | 0.83 | 1.3 | 8.3 lb | 25 lb | 83 lb | 167 lb |
| 0.2 percent | 2 | 1.67 | 2.6 | 16.7 lb | 50 lb | 167 lb | 333 lb |
| 0.3 percent (treatment, no plants) | 3 | 2.5 | 3.9 | 25 lb | 75 lb | 250 lb | 500 lb |
| 0.5 percent | 5 | 4.17 | 6.5 | 41.7 lb | 125 lb | 417 lb | 833 lb |
| 0.6 percent (the dealers' ceiling) | 6 | 5.0 | 7.8 | 50 lb | 150 lb | 500 lb | 1,000 lb |
What dealers, contractors and owners do
The dealers measure in pounds and meters. Hanover Koi Farms says "0.833 (point eight three three) lbs of salt mixed well and completely in 100 gallons of water will give you a salinity of .1%," stages additions "in .1% increments" about a day apart, says salt "may harm certain plants" at 0.2 percent and "will kill most plants" at 0.3 percent, and bans "salt containing YPS (Yellow Prussiate of Soda)." The Midwest Pond and Koi Society publishes "Lb salt to add = (total gallonage of your system/120) multiplied by (desired salt conc. in ppt - current salt concentration in ppt)," says "Do not use salt without a reliable way of testing for it" and "Do not use salt unless you see the nitrite begin to spike up," and runs looser on plants: "Pond plants will tolerate salt levels up to 1.5 lbs/100 gal water." Sacramento Koi sets the top: "0.6% is the highest you would ever want to bring your salt."
The round numbers come from the retailers and contractors. Rail City Garden Center (Salting Your Pond) keeps sales tanks "between 0.3 and 0.5%, which is equal to approximately 3 to 5 pounds per 100 gallons," reports "some Japanese strains of Costia and Trichodina that have built up resistance to salt," says healthy ponds "should not need salt on a continual basis," and that "Trapdoor snails cannot tolerate salt." Hydrosphere Water Gardens matches the club, "2.5 lbs of salt for every 100 gallons of water, or 3 grams per liter," added "over a 3 day period." Kerri Bailey of Olympia Ponds (April 2025) writes "For every 1ppt (or .1%) add 1lb of salt (or about a Cup or 1 ¼ cups) per 100 gallons," keeps planted ponds "below .1%," and answers "Is it Absolutely Necessary to Add Salt? No."
Owners report the plant losses. On Garden Pond Forum, Tam (2007), with a 700 gallon pond, added "1 lb. of salt per 100 gallons over three days" for fin rot: "My fish seems to be getting better, but my hyacinth burned up and died." Lisak1 (2019) keeps "the level at .1%" and says "Above .25% will start to affect plants." crsublette (2014) goes to 6 ppt for bacterial infections in a quarantine tank but says "Do not maintain any long term salinity concentrations in the pond."
Where the tiers differ
The number that circulates without a basis is "3 pounds per 100 gallons equals 0.3 percent." Three pounds in 834 pounds of water is 0.36 percent, or 3.6 ppt. Olympia's 1 pound per 100 gallons for each 0.1 percent actually gives 1.2 ppt, which does no harm at the maintenance level but adds 20 percent extra salt at the treatment level. Cups are worse: The Pond Guy's 2½ cups is about 1.9 ppt with fine salt and less with coarse, which falls between the two levels everyone else uses. Nobody has settled on a plant threshold: the club caps planted ponds at 0.1 percent, Hanover sees harm at 0.2, Lisak1 at 0.25, the Midwest society allows 1.5 pounds per 100 gallons, and Tam lost hyacinth at 1.2 ppt. On duration, Merck says most freshwater fish tolerate 3 grams per liter indefinitely and that a permanent 2 to 3 grams per liter "can minimize some parasitic protistans," while the club, Rail City and Olympia say use it only when needed and the club says year-round salt breeds resistant parasites. Merck is describing what the fish can tolerate, while the koi trade is worried about what the parasites learn. In practice the koi sources treat 0.3 percent as a treatment that lasts days.
What to do
Get the volume right first, because every dose scales with it. Trace the pond in the PondTools app or use the salt itself: read the meter, add a weighed amount, wait 24 hours and read again. Next Day Koi states it as "Pounds of salt X 120 / salinity change = Gallons of Water," with "24 hours for the salt to dissolve." Then pick the level. Use 0.1 percent for nitrite protection while a filter cycles, and only if nitrite is rising. Use 0.3 percent for a parasite found on a scrape, with the plants and snails out and the water changes already planned. Buy salt whose bag lists nothing but sodium chloride, which rules out iodized salt, salt with yellow prussiate of soda, pellets and road salt. Dissolve each third in a bucket of pond water, add it at the skimmer or the falls one day apart for three days, and read the meter before each addition.
Coming back down is arithmetic. Salt leaves only in drained water. Dropping from 3 ppt to 1 ppt takes one change of two thirds of the pond, or three 30 percent changes in a row, which end at about 1.03 ppt but drain 90 percent of the volume in total. Keep the drained water off the lawn and beds, and check your local rules before sending salt water to a storm drain. On 20,000 gallons the 500 pound dose is thirteen 40 pound bags, and a single two-thirds change means draining about 13,300 gallons. Do not add salt after evaporation, because the salt stayed and the level rose. Do not salt a pond with lilies, hyacinth or trapdoor snails above 0.1 percent. Do not leave 0.3 percent in all season.
The PondTools salt calculator
The PondTools salt calculator (also standalone) takes the pond volume in US gallons or liters, the current salt level in ppt from your meter, and a target of 1, 1.5, 3 or 5 ppt, and returns the pounds to add and the number of 40 pound bags, using the Midwest society's gallons divided by 120 formula. A second mode runs the method backwards: enter the pounds you added, the reading before, and the reading after 24 hours, and it estimates the water volume with a range that assumes the meter reads within 0.05 ppt, and compares it with your traced volume if you have one. The water change calculator is the one to open next, since that is how the salt comes back out.
Sources
- Merck Veterinary Manual, Management of Aquarium Fish and Environmental Diseases of Aquatic Animals in Aquatic Systems.
- Weise, C. Salt: The Bottom Line on Salt Use in Ponds, Koi Club of San Diego.
- The Pond Guy, Using Pond Salt in Koi Ponds.
- CupOrGram, Table Salt: 1 Tsp = 6g, 1 Cup = 292g, citing USDA FoodData Central.
- Hanover Koi Farms, Koi Treatments.
- Midwest Pond and Koi Society, FAQs.
- Rail City Garden Center, Sparks, Nevada, Salting Your Pond (PDF).
- Hydrosphere Water Gardens, Bradford, Ontario, Pond Salt, Uses and Benefits.
- Bailey, K. (2025). Pond Salt, Should You Use It?, Olympia Ponds, Tumwater, Washington.
- Sacramento Koi, Rocklin, California, Pond Salinity.
- Next Day Koi, Calculating Pond Water Volume.
- Garden Pond Forum threads: Too Much Salt For Plants? (Tam, 2007); Salt Meter (Lisak1, 2019); How much salt is safe/too much?? (crsublette, 2014).