Ammonia in a koi pond is a water-quality warning, even when the pond looks crystal clear. Koi release ammonia, and uneaten food and other organic waste add to the load. A mature biological filter normally processes this waste fast enough that a hobby test reads zero.
Why Ammonia Appears
- A new or disrupted biofilter.
- More fish or food than the system can process.
- Reduced filter flow or oxygen.
- Decaying material or a problem in replacement water.
Read the Test with pH and Temperature
Many tests report total ammonia, which includes un-ionized ammonia and ammonium. The more toxic un-ionized fraction increases as pH and temperature increase. Low pH does not make an ammonia problem harmless, and deliberately lowering pH is not a safe substitute for fixing it. Ammonia can be harmful without producing a visible cloud or an obvious smell.
What to Do First
- Confirm the result using the kit's instructions and record pH and temperature.
- Reduce or temporarily pause feeding; remove uneaten food.
- Check aeration, circulation, and filter operation.
- Check source water before a controlled partial change, and treat disinfectants as needed.
- Retest and investigate why the filter is not keeping up.
A compatible ammonia-binding product may help temporarily when used according to its label. It does not replace biological filtration, and its interaction with your test method matters. Persistent readings or distressed koi need professional advice.
Prevent Another Spike
Add fish and feeding gradually, avoid cleaning all biological media at once, and test more frequently after equipment changes or treatments. Follow ammonia and nitrite together during recovery.
Source: UF/IFAS: Ammonia in Aquatic Systems. For the wider routine, see koi pond water quality.