View Full Version : Koi Behavior

02-07-2014, 05:16 PM
The first thing to do when observing any of the following behavior is to check your pond or tank water parameters. A lot of the following behavior can be a reaction to the water chemistry being out of whack thus it's important to rule out water chemistry as the cause. It will dictate further action taken.

Know the readings for:
temperature, pH, ammonia, nitrites, nitrates, KH/alkalinity and GH/general hardness

Swings in pH and temperature can distress fish which might cause them to exhibit one or more of the behaviors listed below.
It's important to check the pH at least twice during the day. Once at dawn, once at dusk. If the results have a greater difference than .5, that will often result in flashing as the koi are react to the change.

It is also important to know your source water readings. This is the water with which water changes and filter cleanings are performed.
These readings might explain behavior changes are being seen if they are associated with water changes and filter cleanings.

Most of us have witnessed one or more of the following behavior. Not all are indications that something is wrong but some can be. A combination of them usually indicates a problem.

If the behavior might be a health issue, please refer to the very comprehensive Health Sticky for further guidance:


This is the term used when a fish is scratching itself on a surface in the pond. It could be against the bottom drain, plumbing in the pond, rocks in the pond, etc.
This can be a reaction to water parameters such as a change in pH or an indication of parasites.

This is when a koi jumps and breaks the surface of the pond and dives back in. Some fish jump quite high while others do a more subtle breach. In either case, it's an important behavior to monitor.
This can be a water parameter issue but also can indicate a number of parasites.

This is when the koi is holding one or both of the pectoral fins close to the body. This is not normal activity and usually indicates illness.

Just as it implies, this behavior is when a koi does not swim with the other pond members. They are less active and are often resting on the bottom of the pond. They avoid or ignore other fish. Typically this is an illness.

Listing is when they don't seem to be able to hold themselves upright on the bottom of the pond. Sometimes this is seen in cold water. If the water is warmer and the koi is listing it usually means it is weak. Check for parasites or illness.

Chasing and/or pushing
This behavior typically means the koi are trying to spawn. Often when a new fish is introduced to a pond the other will chase it. In this situation the chasing typically abates within 48 hours. If the pond occupants have been together for a while, it is most likely a prelude to spawning. Watch water parameters since spawnings raise ammonia levels quickly. This can also result in a nitrite spike.

Gasping, mouthing water surface, hovering at waterfall
If a koi is gasping at the surface it usually means they are oxygen deprived. They are doing their best to get oxygen into their gills. Areas like waterfalls have a higher oxygen content which is why koi that are oxygen deprived seek them out.
If this is the case, increase aeration immediately by using a spray bar to break the surface of the water and/or add another air pump with diffusers. Oxygen is paramount to koi health.
If the pond owner is confident that there is enough air being provided and the behavior is with only one koi it's important to check the gills of the fish. It may be that the gills of that fish have sustained damage due to parasites or illness and thus are not processing the oxygen well enough. Immediate action is required to determine the cause and treat the fish. Supplemental air is still recommended since the koi needs it.

Again, for parasite and illness concerns, please refer to the wonderful sticky at the heading of the ER section: