Koi pond water parameters: the levels that matter
Six numbers decide whether your koi thrive — here's the target for each, and when to worry.
Koi are hardy, but they're killed by water chemistry far more often than by disease. Six parameters cover almost everything. Here's where each should sit, the point where you need to act, and how often to check.
The cheat sheet
| Parameter | Target | Act when | Test |
|---|---|---|---|
| pH | 7.0–8.6 | <6.5 or >9.0 | Weekly |
| Ammonia (NH₃/NH₄⁺) | 0 ppm | >0.25 ppm | Weekly (2×/wk in spring) |
| Nitrite (NO₂⁻) | 0 ppm | >0.5 ppm | Weekly while cycling |
| Nitrate (NO₃⁻) | <40 ppm | >80 ppm | Bi-weekly |
| KH (carbonate hardness) | 75–200 mg/L | <50 mg/L | Weekly |
| Salt | 0–0.1% (normal) | per treatment | As needed |
What each one is telling you
- Ammonia & nitrite must read zero. Both are directly toxic — ammonia burns gills, nitrite causes "brown blood disease" (suffocation). Any reading above zero means your biological filter can't keep up; reduce feeding and do a water change. They spike in spring as fish wake and you resume feeding, so test twice weekly then.
- Ammonia gets more toxic as pH and temperature rise. The same ammonia reading is far more dangerous in warm, high-pH water (more of it is the toxic NH₃ form). A "0.25" in a warm summer pond at pH 8.5 is more urgent than the same number in cool water.
- Nitrate is the slow one — it builds between water changes. High nitrate is chronic stress, not an emergency; a water change brings it down.
- KH is the one beginners ignore — and it's the one that kills suddenly. See below.
Carbonate hardness (KH) is your pond's pH buffer. When it drops below ~50 mg/L, pH becomes unstable and can crash overnight — taking the fish with it. It's the single most important "boring" number to watch. (Full fix in the related guide below.)
Gear this guide uses
To read all six numbers and act on the ones that move.
- Cover pH, ammonia, nitrite and nitrate with an API freshwater master test kit — the hobby standard.
- Hold KH steady with baking soda or a pond KH buffer.
- Support the biological filter against ammonia and nitrite spikes with beneficial bacteria.
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Sources
General koi-keeping guidance. Confirm with your own test kit (an API freshwater master kit is the hobby standard) and act on trends, not single readings.