Why your koi pond pH crashes — and the KH fix
The most common sudden koi kill, and the cheapest fix in the hobby.
You test the pond Monday and pH is 8.0. Friday the fish are gasping at the surface and pH reads 6.0. That overnight collapse is a pH crash, and the cause is almost always one number nobody was watching: KH.
What KH actually does
KH (carbonate hardness, or alkalinity) is your pond's pH buffer — the reservoir of carbonates that absorbs the acids constantly produced by fish waste and the biological filter. As long as KH is healthy, pH holds steady. But KH gets consumed over time, and once it runs out, there's nothing left to hold pH up — so it falls off a cliff, fast, often overnight.
Per Pond Informer: KH under 75 mg/L means pH instability is likely, and under 50 mg/L a crash can happen within 48 hours. Test KH weekly — it's the early-warning gauge that prevents the emergency.
The fix: baking soda
Plain baking soda (sodium bicarbonate) raises KH safely and cheaply — it can't push pH dangerously high, it just rebuilds the buffer. The working dose:
~1 teaspoon of baking soda per 50 gallons raises KH by roughly 70 mg/L (ppm).
So a 1,000-gallon pond needs about 20 tsp (~1.5 cups) to lift KH ~70 ppm. Add it dissolved in a bucket of pond water, near the return flow, and re-test after a few hours.
How to do it safely
- If pH has already crashed, raise KH gradually — don't slam it. Bring KH up over a day or two; a sudden large pH swing is itself stressful to fish that are already weak.
- Then keep KH in the 75–200 mg/L range so it never gets close to empty again. If your tap/source water is soft (low KH), you'll need to top up regularly.
- Aerate. Crashing ponds are often low on oxygen too — run a pump/airstone while you correct.
Gear this guide uses
To catch a crash early and rebuild the KH buffer safely.
- Catch low KH before it crashes with a KH test kit, tested weekly.
- Rebuild the buffer cheaply with plain baking soda, or a dedicated pond KH buffer.
- Keep oxygen up during a crash with a pond air pump and airstone.
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Sources
General koi-keeping guidance. In an active crash with fish in distress, prioritize aeration and gradual correction; confirm doses against your own KH test kit.