Koi care: colorful koi swimming in a backyard pond with lily pads

Koi care: colorful koi swimming in a backyard pond with lily pads

By Viki Wu, writes about practical fishkeeping and aquarium care tools

The figures and guidance below were checked against the cited extension and veterinary sources in November 2026 and apply to backyard ponds in 2026.

This guide is for backyard pond owners who already have koi and want a repeatable weekly routine instead of reacting to cloudy water or a fish gasping at the surface. You will learn how to work out your pond's real volume, which four tests to run and how to read them, how big a partial water change should be and how to dose dechlorinator for it, how much to feed by fish weight, and how to shift the routine as the water warms and cools. Everything is worked through on one example pond with the arithmetic shown, so you can swap in your own numbers.

Good koi care is mostly boring repetition: the same tests, at the same time of day, written in the same notebook. That is what makes problems visible early.

Note: this is general educational information for hobbyists, not veterinary advice. If fish are sick, injured or dying, contact an aquatic veterinarian or a fish health diagnostician.

What you need

  • A tape measure and a notebook (or a spreadsheet). You need length, width and average depth, and you need a place to log every test.

  • A liquid test kit for ammonia (total ammonia nitrogen, TAN), nitrite, pH and alkalinity (KH). Texas A&M AgriLife's pond manual notes that simple kits for these are sold by most water garden and aquarium suppliers (Sink et al., Texas A&M AgriLife EWF-014).

  • A pond thermometer that reaches the deep zone, not just the surface.

  • A dechlorinator rated for chloramine if you use city water.

  • A kitchen scale for weighing food portions.

  • A pump or siphon and a hose for partial water changes.

  • About 30 to 45 minutes once a week, plus a two-minute daily look at the fish.

Step 1: Calculate your pond's real volume

Action: Measure the length and width at the waterline and the depth in several places, then average the depths. For a roughly rectangular pond, multiply length × width × average depth in feet to get cubic feet, then multiply by 7.48 to get US gallons (one cubic foot holds about 7.48 gallons). To get liters, multiply gallons by 3.785.

Why it matters: Almost every number in pond care depends on volume. The Texas A&M manual lists dechlorination, feeding, aeration and liming among the jobs that require you to know the pond's surface area, depth and volume (EWF-014). The same manual says most ornamental ponds are 18 to 24 inches deep but should have sections 3 to 5 feet deep to resist winter freezes and give fish a cool retreat in summer, so a single depth measurement in the shallow end will badly underestimate volume.

Worked example (used throughout this guide): a pond measures 10 ft long and 6 ft wide. Depth readings at five points are 1.5, 2.0, 2.5, 3.0 and 3.5 ft, so the average is (1.5 + 2.0 + 2.5 + 3.0 + 3.5) ÷ 5 = 12.5 ÷ 5 = 2.5 ft.

  • Volume in cubic feet: 10 × 6 × 2.5 = 150 ft³

  • Volume in gallons: 150 × 7.48 = 1,122 gallons

  • Volume in liters: 1,122 × 3.785 = 4,247 liters (check in metric: 3.048 m × 1.829 m × 0.762 m = 4.248 m³, about 4,248 liters, so the two methods agree)

Write 1,122 gallons on the first page of your notebook. If your pond has sloped sides, this "average depth" method already accounts for most of the slope.

Step 2: Test ammonia, nitrite, pH and KH at the same time each week

Action: Pick one morning a week and one sampling spot (away from the waterfall return). Test TAN, nitrite, pH and KH, and log the water temperature with each set.

Why it matters: Ammonia is the main nitrogen waste fish excrete, and both ammonia and nitrite are toxic (EWF-014). The Southern Regional Aquaculture Center's koi and goldfish profile recommends keeping un-ionized ammonia and nitrite below 0.05 mg/L, keeping pH as close to 7 as possible (koi tolerate roughly 5 to 9), and keeping dissolved oxygen at least 5 mg/L (SRAC 7201, Species Profile: Koi and Goldfish).

The time of day matters because pH swings. Photosynthesis pushes pH up during the day and respiration pulls it down at night; the Texas A&M manual says pond pH normally cycles between about 6.5 and 9 (EWF-014). That swing changes how dangerous a given ammonia reading is, which leads to the next step.

Step 3: Convert your ammonia reading into the toxic fraction

Action: Your kit reports TAN, which is the sum of toxic un-ionized ammonia (NH3) and much less toxic ionized ammonium (NH4+). Multiply TAN by the fraction for your pH and temperature from SRAC's table to get the toxic part.

Why it matters: SRAC explains that the toxic fraction rises with both temperature and pH, roughly 10 times for each one-unit rise in pH. It also notes that chronic exposure to un-ionized ammonia as low as 0.06 mg/L can cause gill and kidney damage and reduced growth (SRAC 463, Ammonia in Fish Ponds).

Worked example: the example pond reads TAN 0.5 mg/L at 24 °C (75 °F).

  • At a morning pH of 8.0, SRAC's table gives a fraction of 0.0502. Toxic ammonia = 0.5 × 0.0502 = 0.025 mg/L, under the 0.05 mg/L guideline.

  • If the same pond reaches pH 8.4 in the afternoon, the fraction is 0.1171. Toxic ammonia = 0.5 × 0.1171 = 0.059 mg/L, over the guideline and right at the level SRAC links to chronic damage.

Same TAN, same fish, different hour. This is why I always test at the same time of day and treat any measurable TAN as a reason to act. SRAC also stresses that preventing high TAN, mainly through lower feeding rates and good feeding practices, is a better approach than treating it afterward (SRAC 463).

Step 4: Check KH and correct it slowly when it is low

Action: Log KH (total alkalinity, in mg/L or ppm as CaCO3) every week. If it drifts low, raise it with sodium bicarbonate (baking soda) in small portions spread over several days.

Why it matters: Alkalinity is the water's buffer. SRAC's pond chemistry fact sheet says well-buffered water usually stays between pH 6.5 and 9, while low-alkalinity water can swing to dangerously low or high pH (SRAC 464, Interactions of pH, Carbon Dioxide, Alkalinity and Hardness). The Texas A&M manual recommends at least 40 ppm for fish health in ornamental ponds, notes that bacterial biofilters work best at pH 7 to 8 with alkalinity of 50 ppm or more, and warns that biofilters consume alkalinity over time (EWF-014). The Merck/MSD Veterinary Manual describes what happens when buffering runs out in a closed system: pH crashes, the biofilter bacteria die and ammonia climbs (MSD Veterinary Manual, Disorders and Diseases of Fish).

Worked example: SRAC gives a dosing rule of 53 g of sodium bicarbonate per 3,785 liters (1.86 oz per 1,000 gallons) to raise alkalinity by 10 mg/L (SRAC 4502, How to Start a Biofilter). The example pond reads KH 30 mg/L and the target is 60 mg/L, a rise of 30 mg/L, or three 10 mg/L steps.

  • Per 1,000 gallons: 3 × 1.86 oz = 5.58 oz

  • For 1,122 gallons: 5.58 × 1.122 = 6.26 oz

  • Metric check: 3 × 53 g × (4,247 ÷ 3,785) = 159 × 1.122 = 178 g (6.26 oz × 28.35 = 177.5 g, which matches)

I would add about a third of that (roughly 60 g) per day over three days, dissolved in a bucket of pond water, and retest KH and pH before each addition.

Step 5: Do a 10 to 20 percent partial water change and dose dechlorinator for it

Action: Remove 10 to 20 percent of the volume from the bottom of the pond, where debris settles, then refill with treated, temperature-similar water. Dose dechlorinator according to the bottle's label.

Why it matters: Water changes dilute nitrogen waste and restore some buffering. Tap water needs treatment first. The Texas A&M manual says city water should be dechlorinated, that you should ask your utility whether it uses chlorine or chloramine, and that chloramine must be removed chemically with a dechlorinator containing sodium thiosulfate or a similar compound, because aerating or standing the water is not enough (EWF-014). The U.S. EPA notes that chloramines are formed by adding ammonia to chlorine and that more than one in five Americans drinks chloramine-treated water (EPA, Chloramines in Drinking Water). If you fill from a well, the MSD Veterinary Manual warns that a submerged hose can deliver gas-supersaturated water that causes gas bubble disease. Spraying the water in from above lets the extra gas escape (MSD Veterinary Manual).

Worked example for the 1,122-gallon pond:

  • 10% change: 1,122 × 0.10 = 112 gallons

  • 15% change: 1,122 × 0.15 = 168 gallons (about 637 liters)

  • 20% change: 1,122 × 0.20 = 224 gallons

Suppose your dechlorinator label reads "1 mL per 10 gallons of new water." For a 15% change: 168 ÷ 10 × 1 mL = 16.8 mL, rounded up to 17 mL. If your label tells you to dose for the whole pond volume when refilling straight from a hose, follow the label: 1,122 ÷ 10 = 112 mL. Labels differ, so read yours every time you buy a new brand.

Step 6: Check stocking against your pond's carrying capacity

Action: Estimate the adult weight of all fish in the pond and compare it with the Texas A&M rule of thumb. Do this once a season and before buying any new fish.

Why it matters: Texas A&M calls overstocking the most common mistake pond owners make. Its basic guideline is no more than 1 pound of adult fish per 100 gallons. With a pond biofilter and strong aeration (waterfall, air diffuser, submerged plants), that rises to about 2 pounds per 100 gallons. Adult koi in small ponds commonly weigh 4 to 5 pounds, and some get much heavier (EWF-014).

Worked example:

  • Without filtration: 1,122 ÷ 100 × 1 lb = 11.2 lb of adult fish, which is about 2 koi at 5 lb each (11.2 ÷ 5 = 2.2).

  • With biofilter and high aeration: 1,122 ÷ 100 × 2 lb = 22.4 lb, which is about 4 koi at 5 lb (22.4 ÷ 5 = 4.5) or 5 koi at 4 lb (22.4 ÷ 4 = 5.6).

If you have eight young koi in a pond this size, the math says that, once they grow, the pond is overstocked, even if it looks empty today.

Step 7: Feed by body weight, then adjust for water temperature

Action: Weigh the daily ration rather than tossing handfuls. Start from maintenance feeding, watch how fast the food disappears, and scale back as the water cools.

Why it matters: Texas A&M says about 1% of body weight per day keeps pond fish healthy, 3% drives fast growth, and maintenance feeding of 1% a day or 1.5% every other day is usually enough because pond fish also eat algae, insect larvae and zooplankton. Fish should finish the food within about 5 minutes. If food is still floating after 10 to 15 minutes, you are overfeeding: net it out and cut the next meal (EWF-014). Extra feed turns into ammonia, and SRAC recommends cutting feed temporarily whenever TAN rises (SRAC 463).

Worked example: four koi at an estimated 5 lb each = 20 lb = 20 × 453.6 = 9,072 g of fish.

  • Maintenance, 1% per day: 9,072 × 0.01 = about 91 g per day (roughly 3.2 oz)

  • Every-other-day option, 1.5%: 9,072 × 0.015 = about 136 g every second day

  • Growth, 3% per day: 9,072 × 0.03 = about 272 g per day, which only makes sense with strong filtration and warm, well-oxygenated water

Split the daily ration into two or three small meals. SRAC notes that goldfish and common carp, the species koi were bred from, lack a true stomach and benefit from multiple feedings each day (SRAC 7201).

Step 8: Shift the routine with the seasons

Action: Use the deep-zone water temperature, not the air temperature, to decide how much to feed and how often to test.

Why it matters: SRAC gives an optimal range for koi of about 65 to 75 °F (18 to 24 °C). Koi survive from about 32 to 95 °F, but outside the optimal range they eat and grow less (SRAC 7201). A practical seasonal plan:

  • Warm season (water about 65 to 85 °F): full maintenance ration, weekly tests, weekly 10 to 20% change. Above 85 °F, Texas A&M advises feeding sparingly and in the early to mid-morning, because warm water and heavy feeding can lead to oxygen depletion (EWF-014).

  • Spring and fall transitions: test nitrite two or three times a week. SRAC notes that nitrite spikes in ponds are more common in fall and spring, when changing temperatures disrupt plankton and bacterial activity (SRAC 462, Nitrite in Fish Ponds). Ammonia also tends to rise in fall and winter, as algae die back and cooler water slows bacterial nitrification (SRAC 463).

  • Cold season: Texas A&M says feeding must be reduced or stopped in the cooler months, when fish metabolism slows (EWF-014). In my own routine I cut the ration in steps as the deep-zone reading falls through the 60s °F and stop feeding entirely once it stays in the low 50s or colder, then restart with small meals when it climbs back.

Also keep an eye on disease timing. SRAC notes that koi ulcer disease is most common from 55 to 75 °F and after spawning or handling, and that koi herpesvirus (KHV) occurs most often at 64 to 81 °F (SRAC 7201). Spring is when I watch behavior most closely.

Step 9: Write it all down and look for trends

Action: For each weekly entry, record the date, time, water temperature, TAN, nitrite, pH, KH, gallons changed, dechlorinator dose, grams fed per day and any notes on behavior.

Why it matters: One reading tells you very little. Four weeks of KH sliding from 80 to 50, or TAN creeping from 0 to 0.25 as fish grow, tells you what to change before fish show symptoms. The MSD Veterinary Manual names poor water quality, overcrowding, stress and failure to quarantine new fish as the root of many fish disorders (MSD Veterinary Manual), and all four show up in a log before they show up on a fish.

Common mistakes and how to fix them

  • Guessing volume from the shallow shelf. A 2 ft reading in a pond that averages 2.5 ft understates volume by 20%, so every dose and stocking figure comes out wrong. Fix: average depth readings from at least five points, as in Step 1.

  • Reading TAN without pH. 0.5 mg/L can be fine at pH 7.4 and harmful at pH 8.4. Fix: always log pH and temperature with ammonia, and convert TAN using SRAC's table.

  • Topping up evaporation instead of changing water. Adding water without removing any lets acids build up and buffering fall. The MSD manual describes this exact pattern leading to pH collapse (MSD Veterinary Manual). Fix: remove water before refilling, and track KH weekly.

  • Using a chlorine-only product on chloraminated water. Fix: call your utility or check its water quality report, and buy a dechlorinator labeled for chloramine (EWF-014).

  • Feeding to the "begging" response. Koi always look hungry. Fix: weigh the ration and use the 5-minute rule. Net out food left after 15 minutes.

  • Stocking for today's fish size. Fix: run the carrying-capacity math with adult weights before you buy.

  • Dumping in a big correction. Swinging KH or pH quickly stresses fish. Fix: split chemical additions over several days and retest between doses.

FAQ

How often should I test my koi pond?

Weekly is a solid baseline for TAN, nitrite, pH and KH. Test two or three times a week during spring and fall transitions, after adding fish, and any time fish behave oddly. SRAC recommends checking pond nitrite two to three times a week in fall and spring and at least weekly the rest of the year (SRAC 462).

How big should a weekly koi pond water change be?

10 to 20 percent of the total volume is a manageable routine. For the 1,122-gallon example, that is 112 to 224 gallons. Increase the volume or frequency if ammonia or nitrite are measurable, and always treat tap water first.

What should I do if ammonia shows up?

Stop or reduce feeding for a few days, do a partial water change with treated water, and check that the filter and aeration are working. SRAC recommends a temporary feeding cut until TAN falls (SRAC 463), and Texas A&M lists reducing feed, flushing with good water and servicing filters as the fixes for ammonia and nitrite build-up (EWF-014).

How many koi can my pond hold?

Start with about 1 lb of adult fish per 100 gallons, or about 2 lb per 100 gallons with a biofilter and strong aeration (EWF-014). Divide by an expected adult weight of 4 to 5 lb per koi.

Summary checklist

  • Measure length, width and average depth, then calculate gallons (ft³ × 7.48) and liters (gallons × 3.785).

  • Test TAN, nitrite, pH and KH weekly, at the same time and place.

  • Convert TAN to un-ionized ammonia with SRAC's pH and temperature table, and aim for under 0.05 mg/L.

  • Keep KH at or above roughly 40 to 50 ppm, and raise it with baking soda in small daily doses.

  • Change 10 to 20% of the water weekly and dose a chloramine-rated dechlorinator by the label.

  • Check that adult fish weight stays within 1 to 2 lb per 100 gallons.

  • Feed about 1% of body weight per day, finished within 5 minutes, and cut back as water cools.

  • Log everything and act on trends, not single readings.

References

Tags: koi care, koi pond, pond maintenance, water quality, fishkeeping, how-to