Koi Pond Water Chemistry: pH, Ammonia & Nitrates Explained
What every test kit number means, safe ranges for SoCal koi ponds, and exactly how to fix each parameter when it goes wrong.
Read Guide →Why Koi Feeding Follows Water Temperature
Fish are poikilotherms ("cold-blooded"), meaning they have very little ability to control their own body temperature, and their body systems work best within a temperature range that differs by species[1]. Put simply, a koi's body is the same temperature as the pond water it lives in.
As water cools, a koi's ability to digest food falls[4]. Measure the water itself. The air temperature and the weather forecast are not a substitute.
Very warm water is a problem too. Hikari, a koi food maker, says koi metabolism starts to decline above 80°F, just as it does below 65°F[5]. Warm water also holds less oxygen, while every meal adds to the pond's waste load.
Feeding by Water Temperature
The table combines published guidance. Bands differ between sources, and the main charts, Hikari's included, are written with cold-winter climates in mind (see "Southern California" below). Treat the numbers as guides, not hard cutoffs. Hikari itself calls its temperatures "examples only"[4].
| Water temperature | How often | What to feed | Notes |
|---|---|---|---|
| Below 50°F | Stop feeding | Nothing | Stop feeding below 50°F until spring. Hikari agrees you may stop until spring at 41–50°F, and only permits a tiny feed (what they eat within a minute or so) up to 2–3 times a week if koi are active; it says to stop completely below 41°F[4]. |
| 50–59°F | Up to 2 times a day[4]; below 55°F, anywhere from once a day to about once a week | Easily digested, low-protein food such as wheat germ[4] | Feed in the warmest part of the day. Skip feeding if koi don't readily eat[4]. |
| 59–68°F | 1–2 times a day[4] | Move up to higher-protein food once water is steady around 60°F | When switching, mix the two foods half-and-half for about a week. |
| 68–80°F | 2–4 times a day, in smaller amounts[4] | Growth, colour or staple foods[4]; Hikari names 68–78°F as the best range for higher-protein growth and colour diets[5] | Koi are at their most active[4]. |
| About 80–86°F | Reduce | Hikari suggests switching to an easier-to-digest food such as wheat germ above 80°F[5] | Reduce the quantity from about 82°F, and scale back further above 85°F. |
| 86°F and above | No more than 2 times a day (small koi up to 4)[4] | Small amounts of easily digested food[4] | Koi can lose their appetite. Shade the pond[4]. |
Where sources disagree
- The stop-feeding point. This site uses about 50°F as the point to stop feeding. Hikari supports stopping at that point too: at 41–50°F it says you may stop until spring, and it only permits very small, optional feeds there if koi are active[4]. Koi will eat actively down to about 50°F before their appetite may stall.
- When to start cutting back in heat. The figures run from 80°F (Hikari's summer article[5]) through 82°F and 85°F to 86°F (Hikari's chart[4]). There is no single temperature at which a pond suddenly becomes unsafe.
- Hikari against Hikari. Hikari's summer article says metabolism declines above 80°F[5], while its feeding chart calls 68–86°F the most active range[4].
How Much and How Often
The "few minutes" rule
Every source we found sizes a meal by how long the koi take to eat it, but they give different times:
- Hikari: only what the fish eat "within a few minutes"[5].
- Other guides: an amount eaten in 3–5 minutes, or in 5–10 minutes.
A veterinary reference gives a weight-based guide for fish in general: about 3% of body weight a day for adult maintenance, and up to 5% for young fish[2]. That figure comes from the Merck manual's aquarium-fish chapter and is not specific to koi.
Start with a handful. If the koi clear it and still come up, feed a bit more. If food is still floating, net it out and wait a few hours before feeding again.
How often
In warm, active months, small meals several times a day work best: up to four, or one every three to four hours. Hikari advises adding meals rather than adding food: increase the number of feedings, not the total amount[5]. Hikari also advises against feeding within an hour of sunrise or sunset[4]. Low oxygen is most common early in the morning in outdoor ponds[1].
Choosing a Food
Low-protein and wheat-germ foods (cool water)
As water cools, sources recommend "easily digested" foods, with wheat germ named most often[4]. Hikari also recommends them when restarting feeding in spring, increasing the amount little by little[4].
High-protein growth foods (warm water)
Fish foods are typically very high in protein, and fish excrete the waste from that protein as ammonia through their gills[1]. More protein means more growth, and also more load on the filter. The more you feed, the more the koi grow and the more waste they produce.
Colour foods
Koi cannot make their own red or yellow pigments, so they rely on carotenoids in their diet. Colour foods work best above 18°C (about 64°F) and in warm, stable water. Overusing synthetic carotenoids may turn the white skin yellow.
Treats
Peas, lettuce and watermelon make good treats in moderation. Avoid bread, cereals, processed snacks and high-carbohydrate foods. Hikari warns that mixing many foods can upset the nutrient balance of a formulated diet[5]. Fish also need vitamin C in their feed, and most fish foods should contain a stabilized form[2].
Overfeeding and Water Quality
Overfeeding is a common cause of rising ammonia[1], and excessive feeding is a common cause of low oxygen in ponds[1]. Leftover food pollutes the water and loses its water-soluble vitamins as it dissolves. Remove leftovers right away[4]. For ammonia readings and what to do about them, see our water chemistry guide.
Hot-Weather Cautions
- Every meal adds to the pond's biological load, so heat calls for more care, not more food.
- If koi seem to be in respiratory distress, fix the pond first. Feeding can wait.
- Reduce or stop feeding if koi lose interest or behave oddly, because this may signal low oxygen, and don't feed at all when oxygen is very low.
- After a storm or during a heatwave, skip a meal or two.
Special Situations
Illness or sudden changes
Hikari advises stopping or reducing feeding after a sudden temperature change, unstable water quality or illness until things are corrected[4]. When ammonia or nitrite is a problem, the Merck manual says feeding should be stopped or substantially reduced until it is fixed[1].
New arrivals
Don't feed newly arrived koi for 24 hours, then feed easily digested food daily during quarantine. Hikari advises feeding less in a new pond until the filter has fully cycled[4].
Before transport
Koi going to a show should be "purged" (not fed) for at least a week beforehand, to reduce ammonia build-up in the bag. That advice is written for show transport. We did not find a primary source giving a fasting period for routine shipping.
Storing Koi Food
Fish foods are high in protein and oil, which go rancid. Keep dry food in a pest-proof container in a cool, dry place[2][3]. Heat and humidity cause the food to break down, grow mould and go rancid[2]. Sources differ on how long food keeps:
| Source | Opened food at room temperature | Freezing |
|---|---|---|
| Merck Veterinary Manual[2] | Throw out 2 months after opening | Frozen dry food: throw out after 6 months |
| UF/IFAS Extension[3] | Never keep on hand more than 3 months; buy at least monthly | Freezing is acceptable; refrigeration is not recommended |
A sealed, unopened bag keeps much longer than an opened one, so check the date on the bag. Repackaging food into smaller daily portions also helps.
Southern California
Most published feeding calendars assume a cold winter. Hikari's chart, for example, was written "to help you in chilly times"[4]. In SoCal, the bigger feeding decisions usually come in summer.
- Air temperatures (NOAA 1991–2020 normals). The average August high is about 97°F at the Riverside Fire Station 3 weather station, 85°F at the Santa Ana Fire Station weather station and 77°F at Los Angeles International Airport. The average January low is about 43°F at the Riverside station and about 49°F at both the Santa Ana and LAX stations[6]. These are air temperatures, not pond water temperatures.
- Pond water. We found no university or government data on SoCal pond water temperatures. Typically, coastal ponds rarely get warm enough to stress fish, inland ponds in Riverside and San Bernardino see summer water above 85°F, and Southern California ponds rarely drop below 40°F.
In practice, this means:
- Coastal ponds may never reach the true stop-feeding range. They may spend winter in the cool, wheat-germ bands of the table instead. Go by your thermometer. The 50°F stop rule still applies whenever the water gets there.
- Inland ponds are more likely to spend summer weeks in the reduce-feeding bands above 80–86°F[4]. They are also where you are most likely to need the hot-weather cautions.
FAQ
Do koi need to stop eating in a Southern California winter?
Only if the water gets cold enough. The usual stop point is around 50°F[4]; Hikari also allows tiny, optional feeds down to 41°F if koi are still active[4]. Many coastal ponds may stay above that, but measure the water to be sure.
My koi act hungry all the time. Should I feed more?
Not necessarily. Size each meal to what they finish in a few minutes[5], and remove leftovers[4]. Overfeeding is a common cause of ammonia problems[1].
Is it OK to keep feeding growth food in August?
At moderate temperatures, yes[4][5]. Above about 80–86°F, sources advise feeding less and switching to an easier-to-digest food[4][5].
How long does an open bag of koi food last?
The Merck manual says 2 months at room temperature[2]. UF/IFAS says no more than 3 months on hand[3].
Should I feed new koi right away?
Wait 24 hours, then feed small amounts of easily digested food.
Sources
- Merck Veterinary Manual, "Environmental Diseases of Aquatic Animals" (UF veterinary authors; page last updated Jun 2026)
- Merck Veterinary Manual, "Nutritional Diseases of Fish" (aquarium-fish chapter; Petty, Francis-Floyd, Yanong, UF)
- UF/IFAS Extension VM114 "Fish Nutrition," Ruth Francis-Floyd DVM (2002, reviewed 2011; PDF hosted on a Texas A&M extension server)
- Hikari Sales USA (feed maker), "Feeding Your Coldwater Koi – The Basics"
- Hikari Sales USA, "Koi in Summer"
- NOAA NCEI U.S. Climate Normals 1991–2020, monthly CSVs: Riverside Fire Stn 3, Santa Ana Fire Stn, Los Angeles Intl AP