When to stop feeding koi: a water temperature chart

Most sources tell you to switch koi to a wheat germ or cold-water food somewhere between 60°F and 50°F and to stop feeding somewhere between 50°F and 40°F, and they disagree by about ten degrees on both lines. The safe reading is to cut back below 60°F, feed small amounts of wheat germ food less and less often through the 50s, and stop once the water at the bottom of the pond holds below 50°F. Food eaten in cold water still turns into ammonia, and the bacteria that clear it slow down as the water cools, so overfeeding in fall shows up as ammonia trouble weeks later.

The chart, one source per row

Each row is one source's own rule. Feed makers write it for their own food, and dealers and forum posters report what they do.

What the authoritative sources say

Few authoritative sources give a koi feeding temperature at all. Michigan State University Extension says (in a passage we read through a search snippet, because the page blocked our fetch) "Stop feeding the fish when water temperatures reach 45 degrees Fahrenheit. Even if the fish appear active they are entering a dormant state and will not be able to adequately process any food given to them." Texas A&M's EWF-014, by extension fisheries specialist Todd Sink with Jessica Gwinn and Hannah Gerke, gives amounts instead: "Maintenance feeding at 1% a day or 1.5% every other day is typically sufficient," feeding "must be reduced or even ceased during cooler winter months when their metabolism is not active," and "If feed remains within 10 to 15 minutes from the time food is offered, you are over feeding them."

Dr. Jessie Sanders, a board-certified fish veterinarian in California, writes that below 55°F "Koi metabolism slows down greatly," and that from 55 to 65°F the pond should stay in that range "for 5 hours minimum" for the fish to digest a meal. The feed makers set the line for their own food. Hikari stops at 41°F, and Aquascape's cold-water label says "It is NOT recommended to feed your fish in water temperatures below 50 degrees." Aquascape's feeding pages refused our fetches, so its 60°F switch point comes from a search snippet of its fall feeding page.

Why feeding in cold water leads to ammonia trouble

Fish make ammonia from the protein they eat. SRAC Publication 4603 (Hargreaves and Tucker, 2004) says "protein in feed is the ultimate source of most ammonia in ponds where fish are fed," and food the fish leave behind adds more as it decomposes on the bottom. Texas A&M adds that "the demand on the filter arises from the amount of feed placed into the ornamental pond, not the volume of the pond."

Meanwhile the bacteria that convert ammonia slow down. SRAC 4603 says "During winter, low temperature suppresses microbial activity." The FAO's small-scale aquaponics manual (Somerville and others, 2014) says "The optimal temperature range for nitrifying bacteria is 17-34 °C," which is about 63 to 93°F, that below it "the productivity of the bacteria will tend to decrease," and that the nitrite-clearing bacteria handle cold worse, so "during colder periods nitrite should be more carefully monitored."

Withholding food does not clear the problem quickly either. SRAC 4603 says "sharp reductions in feeding rate have little immediate effect on ammonia concentration," and that high ammonia in late fall and winter "can be related to the total amount of feed added to a pond during the preceding growing season." Those findings come from commercial catfish ponds, but the chemistry carries over.

Here is the arithmetic. SRAC Publication 451 (Losordo, Masser and Rakocy, 1998) says that for a 32 percent protein feed, "Approximately 3 percent of the feed becomes ammonia-nitrogen." Say your fish weigh 10 pounds in total, or about 4.5 kilograms. Texas A&M's 1 percent maintenance rate is about 45 grams of food a day, which makes about 1.4 grams of ammonia-nitrogen. In a 3,000 gallon pond (11,356 liters) that adds about 0.12 mg/L a day, or about 0.84 mg/L over a week, if the bacteria and plants take none of it up. At the 3 percent growth rate, the same fish get about 136 grams a day, which adds about 0.36 mg/L a day and about 2.5 mg/L over a week. A 1,000 gallon pond gets three times those figures, and a 20,000 gallon pond gets about a seventh of them. A mature filter clears that load daily in summer and less of it in cold water, so a summer feeding habit carried into October builds up a reading.

Cold water softens the danger in one way. At pH 8.0, the Emerson equation that the PondTools ammonia calculator uses puts about 1.8 percent of a reading in the toxic free form at 50°F and about 5 percent at 75°F. The risk moves to spring, when the water warms, the free share of whatever has built up nearly triples, and the bacteria are still waking up. Our post on new pond syndrome and ammonia water changes covers what to do once a reading is high.

How to measure water temperature at depth

Read the water where the fish sit, near the bottom of the deepest part of the pond. The USGS explains why the bottom differs in cold weather: "Since water at about 39°F (4°C) is denser than water at 32°F (0°C), in lakes and other water bodies the denser water sinks below less-dense water." The Pond Guy recommends "your submersible thermometer to monitor the water temperature throughout the seasons." Chris G at Pond Informer writes that "deeper water will be a different temperature than surface water," recommends a probe at least one meter long for a typical koi pond, and suggests checking "both at midday (when temperatures are highest) and at night or in the morning when temperatures are cooler."

Hikari says to feed cool-water fish "between 10:00 am and 2:00 pm." Feed then, but decide whether to feed at all from the cold morning reading. The Pond Guy restarts only when the thermometer "consistently" reads high enough, so one warm spring afternoon is not enough.

What contractors and experienced owners do

The dealers set their lines higher than The Pond Guy and Hikari do. Hydrosphere Water Gardens in Ontario feeds "nothing" below 48°F, saying that feeding in water that is too cold "will do more harm than good." Kloubec Koi Farm in Iowa starts wheat germ food in spring at 55°F and moves to high-protein food once the water "stabilizes at 60°F." On a 2018 Garden Pond Forum thread, Somethin' Fishy gave the common 60 and 50 degree rule, Rich8888ri stressed that these are "WATER TEMPS not Air temp," and other owners went by the fish. Gemma wrote "I stop feeding when the fish stop asking for food," Jhn wrote "If they want to eat feed them," and the original poster, Stingray, tapered until the fish "refused the very small amounts."

Where the sources differ, and why it matters

The stop line runs from 40°F at The Pond Guy to 50°F at Aquascape, with Hikari at 41°F, Michigan State at 45°F and Hydrosphere at 48°F in between. The wheat germ line differs more. The Pond Guy keeps protein food down to 50°F, while Hikari, Aquascape, Kloubec and Hydrosphere switch somewhere between 59 and 65°F. Sanders, the only veterinarian here, sets no hard stop. No source we found ties its line to a measured digestion or ammonia study.

Pond sites widely repeat figures like the ones Pondmaster posted on Garden Pond Forum in 2010, that nitrifying bacteria growth "is decreased by 75% at 46-50° F" and "No activity will occur at 39° F." We found no authoritative source for those exact numbers. The FAO says activity falls below its optimal range without putting a percentage on it, so treat the forum figures as a rough picture.

What to do

1. Put a probe thermometer at the bottom of the deep zone and read it each morning from late summer on.
2. When the morning reading holds below about 60°F, mix wheat germ food into the summer food and shift toward it, as Hydrosphere suggests.
3. Feed only what the fish clear in three to five minutes, dropping to once a day and then every few days through the 50s. Net out anything left after 15 minutes.
4. Stop when the morning reading holds below 50°F. If you follow Hikari or The Pond Guy and feed down into the 40s, keep it to a pinch two or three times a week.
5. Test ammonia and nitrite before you stop for the winter and again in spring. Restart with wheat germ food after several mornings above 50°F.
6. Do not feed fish that sit on the bottom ignoring food, and, as Kloubec warns, do not feed when oxygen is very low.

For fishing ponds and game fish, which are fed by the acre, see our post on feeding pond fish.

The PondTools stocking and ammonia calculators

The PondTools stocking calculator (also as a standalone page) takes the pond volume in gallons, the surface area in square feet and the count and current length of each kind of fish (medaka, comets and shubunkins, fancy goldfish and koi), and checks the load today and at adult size, which is what sets how much food goes in. The PondTools ammonia calculator (also as a standalone page) takes your 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, so you can rerun the same reading at spring temperatures. A pond traced in the PondTools app fills in the volume and surface area.

Sources

• Winter visits your backyard pond, Michigan State University Extension (quoted from a search snippet)
• EWF-014 Ornamental Ponds & Water Gardens in Texas (Sink, Gwinn and Gerke), Texas A&M AgriLife Extension Service
• SRAC Publication No. 4603, Managing Ammonia in Fish Ponds (Hargreaves and Tucker, 2004), Southern Regional Aquaculture Center
• SRAC Publication No. 451, Recirculating Aquaculture Tank Production Systems (Losordo, Masser and Rakocy, 1998), Southern Regional Aquaculture Center
• Nitrifying bacteria and the biofilter, from Small-scale Aquaponic Food Production, FAO Fisheries and Aquaculture Technical Paper 589 (Somerville and others, 2014), reproduced by FarmHub
• Water Density, USGS Water Science School
• How to Feed Koi, Dr. Jessie Sanders, Aquatic Veterinary Services
• Feeding Your Coldwater Koi, Hikari Sales USA
• Fish Feeding Tips, The Pond Guy
• When should I start feeding my koi fall fish food?, The Pond Guy
• How to Properly Feed Fish in the Fall, Aquascape (quoted from a search snippet)
• Aquascape Premium Cold Water Fish Food Pellets, The Pond Shop
• My Koi Are Hungry, Kloubec Koi Farm
• Feeding Koi at Different Temperatures, Hydrosphere Water Gardens
• Pond Water Temperature Guide, Pond Informer
• When do I stop feeding? (Stingray, Somethin' Fishy, Gemma, Jhn, Rich8888ri, 2018), Garden Pond Forum
• Water temperature & nitrifying bacteria (Pondmaster, 2010), Garden Pond Forum