Koi pond fish killed, so what happened?

I was gone for most of Memorial Day. When I got home, something strange was happening in my pond. Many of the goldfish and some of the koi were behaving abnormally.

They were lethargic, and some were staying directly under the waterfalls in an apparent attempt to get more oxygen. Before the next morning, a 21-inchlong koi, a foot-long koi and seven large goldfish were dead.

I had to be part historian, part detective and part coroner to try to figure out what happened. The historian in me determined that there were no sick fish or signs of trouble before Memorial Day. However, a severe thunderstorm did move through during the afternoon, with rain, high winds and marble-sized hail.

A lot of shoreline plants and nearby landscape plants were damaged. Lots of leaves and parts of plants were floating in the pond.

The detective discovered that some of the neighbors held family events in their nearby yards.

Could some of them have sprayed their yards for mosquitos? So far, no one has said they did any spraying.

The coroner noted that none of the fish had any outward signs of injury. It appears the hail did not hit the fish.

Some of the goldfish had been breeding in the morning and appeared healthy.

So, what are the possible causes of this fish kill? First, poison in the water. I did add water to the pond the night before because it was getting low. Occasionally, the local municipal water supply changes the wells they use to supply water.

The water may have had a higher nitrite level than usual. The legal limit for nitrite in drinking water is much higher than the level necessary to harm fish. I should have tested the water before adding it as well as tested the pond water after noticing the fish in distress, but I didn’t think about it until a day later.

Maybe the plant material knocked into the water by the storm released some toxic chemicals. There were a lot of leaves in the pond.

Maybe lightning from the storm increased the amount of nitrate in the rainwater. Nitrate fertilizes plants, but it is toxic to fish.

Lightning can kill fish in lakes, but the neighbors didn’t think any lightning struck close by; no nearby trees appeared to be injured. Speaking of electricity, submersible pumps can wear out and cause electricity to leak out into the water, harming fish; however, this pond uses an out-of-water pump that can’t harm the fish.

The only sign there was a chemical cause to the problem was the foam that formed near the waterfalls, during which time the fish appeared most stressed. I did a huge water change during the night of Memorial Day and the level of foam decreased.

One of the odd parts of this problem was that two koi over 2 feet long, one 8-inch koi and several goldfish survived with only minor irritation during the episode and appear unharmed more than a week later.

One of the lessons that can be learned here is that water should be checked before being added in large amounts to a pond. Nitrite and nitrate are toxic to fish, and test kits are easily purchased at pet stores. Small water changes of less than 10% will increase nitrite and nitrate, but in an established pond, the beneficial bacteria will consume both and they won’t harm the fish.

Another lesson is that all debris should be removed from a pond as soon as possible. This pond is covered by a net in the fall to prevent leaves from filling it.

A final lesson is that fish are very sensitive to airborne chemicals, especially insecticides. Weedkilling herbicides can also kill fish.

If anyone in your neighborhood has a fishpond, be very careful to apply chemicals when the wind won’t blow them toward the pond.

One of the lessons that can be learned here is that water should be checked before being added in large amounts to a pond. Nitrite and nitrate are toxic to fish, and test kits are easily purchased at pet stores. Small water changes of less than 10% will increase nitrite and nitrate, but in an established pond, the beneficial bacteria will consume both and they won’t harm the fish.