What the numbers on your test kit actually mean — and what to do about them
Koi live entirely in the water around them. They breathe it, absorb it through their skin, and rely on it to regulate their body temperature. When the chemistry is off, even slightly, fish feel it immediately — even if you can't see anything wrong yet.
The good news is that water chemistry follows predictable rules. Once you understand the key parameters and what they mean, keeping your pond healthy becomes much more intuitive. You don't need to be a scientist — you just need to know what to test, what the numbers mean, and what to do when something is off.
pH measures how acidic or alkaline your water is on a scale of 0–14. Pure neutral water sits at 7.0. For koi, the ideal range is 7.0–8.0, with 7.2–7.6 being the sweet spot.
pH that's too low (acidic) stresses koi and can cause their protective slime coat to break down, making them vulnerable to disease. pH that's too high (alkaline) makes ammonia far more toxic — a reading that's manageable at pH 7.0 can become deadly at pH 8.5.
What affects pH: decaying organic matter like leaves and uneaten food drives pH down over time. Limestone rocks, certain gravels, and tap water often push pH up. pH also naturally rises during the day as plants photosynthesize and falls overnight — a swing of 0.5 or more in 24 hours is stressful for fish.
To raise pH: add crushed coral or oyster shell to your filter. To lower pH: perform a partial water change, remove organic debris, and reduce plant mass.
Ammonia is produced constantly by your koi through their gills, urine, and decomposing waste. In a healthy, cycled pond, beneficial bacteria convert ammonia into less harmful compounds almost as fast as it's produced. In a new pond or an overstocked one, ammonia can build to dangerous levels quickly.
The safe level for koi is 0 ppm. Any detectable ammonia is a cause for concern. At 0.5 ppm, koi will show signs of stress — clamped fins, hovering near the surface, reduced appetite. At 2 ppm and above, gill damage occurs. At high levels, ammonia is rapidly fatal.
If you detect ammonia: do a 25–30% water change immediately using dechlorinated water. Reduce or stop feeding — unprocessed food adds more ammonia. Check your filter is running correctly and hasn't been cleaned too aggressively, which can wipe out your beneficial bacteria.
Nitrite is produced when beneficial bacteria convert ammonia. It's less immediately toxic than ammonia but still very harmful. Nitrite interferes with the ability of koi blood to carry oxygen — even with plenty of dissolved oxygen in the water, fish with nitrite poisoning effectively suffocate.
Safe level: 0 ppm. Anything above 0.25 ppm warrants action.
Nitrite spikes are most common in new ponds during the cycling process, after a filter crash, or after adding too many fish too quickly. Signs of nitrite poisoning look similar to ammonia toxicity — gasping at the surface, lethargy, and pale gills.
Emergency treatment: add pond salt at 0.1–0.3% concentration. Salt temporarily blocks nitrite uptake through the gills and buys you time while you do water changes and investigate the root cause.
Nitrate is the end product of the nitrogen cycle and is far less toxic than ammonia or nitrite. Koi can tolerate nitrate levels up to around 40–80 ppm without obvious distress, but chronic high nitrate suppresses immune function and makes fish more susceptible to disease over time.
Ideal level: below 40 ppm. Many experienced keepers aim for below 20 ppm.
Nitrate accumulates gradually and the only way to remove it is through water changes and plant uptake. This is why regular partial water changes are so important — not just to dilute ammonia, but to keep nitrate in check over the long term.
If your nitrate is creeping up between water changes, consider adding more plants like water hyacinth or water lettuce, which are excellent nitrate absorbers, or increasing the frequency of your water changes slightly.
KH (carbonate hardness) and GH (general hardness) are the parameters that confuse most beginners, but they're actually straightforward once you understand their roles.
KH, also called alkalinity, measures the buffering capacity of your water — its ability to resist pH swings. This is critically important for koi ponds. A low KH means your pH can crash suddenly, which is extremely dangerous. Ideal KH for koi is 80–120 ppm, or around 4.5–6.7 dKH. If your KH is below 4 dKH, your pH is at risk of crashing, especially overnight.
GH measures the total concentration of dissolved minerals — primarily calcium and magnesium. These minerals are essential for koi health, proper scale development, and mucus coat integrity. Ideal GH for koi is 100–200 ppm, or 5.6–11.2 dGH.
To raise KH: add crushed coral, limestone chips, or a commercial KH buffer. To raise GH: use a commercial GH booster or add calcium and magnesium supplements.
Koi are cold-blooded, meaning their metabolism, immune system, and digestion all run at different speeds depending on water temperature. Temperature isn't just a comfort issue — it dictates almost everything about how you care for your fish.
At 65–75°F: koi are in their prime zone. Active, hungry, and their immune systems are fully functional. Feed a quality staple food 2–3 times daily.
At 55–65°F: metabolism slows. Switch to a wheat germ based food which is easier to digest at lower temperatures. Feed once daily.
Below 55°F: reduce feeding to every few days, very small amounts. Below 50°F, stop feeding entirely. Undigested food in a koi's gut at cold temperatures can cause serious internal problems.
Above 80°F: oxygen levels in water drop, stress increases. Make sure your aeration is excellent during heat waves and avoid feeding heavily in the hottest part of the day.
Fish breathe oxygen dissolved in water, and koi are no different. Healthy koi ponds should maintain dissolved oxygen levels above 6 ppm, with 8 ppm or higher being ideal.
Oxygen enters pond water through surface agitation — waterfalls, fountains, and air stones all help. Warmer water holds less oxygen than cooler water, which is why summer nights can be particularly dangerous. If all your fish are gasping at the surface first thing in the morning, low dissolved oxygen is the most likely cause.
Signs of low oxygen: fish at the surface, especially overnight or early morning, reduced appetite, and lethargy. Emergency fix: add an air stone or increase surface agitation immediately.
Algae can cause oxygen crashes overnight — during the day algae produces oxygen, but at night it consumes it. A heavy algae bloom combined with warm temperatures is a recipe for a sudden overnight oxygen crash.
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