Warm weather and Koi can be a lethal combination. When you have had a good few days of extremely warm weather often we are called out for a Koi pond mass death due to asphyxiation. We’re very pleased by not being called out for some time now and hopefully this means that everyone is now well aware that when you are at altitude, dealing with warm weather and feeding heavily to an overstocked Koi pond that you have created the perfect recipe for mass mortality of your large Koi.

For those of you new to all this and as a refresher to everyone else… Koi need oxygen. Big Koi need plenty oxygen – loads of it in fact. As water warms up it carries less dissolved oxygen – yes – eventually it gets so warm that the quantities of oxygen it carries are not enough to sustain the demand from a large Koi resulting in the death of said large Koi. Note that these temperatures are typically much higher than Koi can withstand – meaning that they will be long dead owing to the temperature rather than the lack of oxygen in the water.

OK, so problem solved right?

Nope!

We love doing that!

It makes everyone think. If you consider this carefully – the above assumes that you are dealing with water that is completely saturated with oxygen. In other words, you assume that as much oxygen as the water can dissolve, it has dissolved and that all of this dissolved oxygen is available to the fish.

This is a Big and Nasty Assumption to make! Filters take oxygen out of the water (unless you are using something like our active bed media filters of course). Bacteria in the water take oxygen out of the water. Slowly circulating ponds don’t get enough water from the bottom of the pond to the top of the pond where oxygen transfer can take place. Plants take oxygen out of water at night… Etc…

When moving some fish for a customer recently in what has to be one of the biggest Koi ponds in the country that hasn’t run in years, we encountered about two feet of pure sludge at the bottom of the pond. Catching fish in this pond was easy. All we did was stir up this toxic sludge a bit and not ten minutes later the fish were on the surface completely asphyxiated owing to the massive oxygen uptake by the bugs in the sludge. Oxygen thieves we call them - and it can happen to you if you don't keep your filters and pond clean!

We then scooped the fish up into waiting 3m porta ponds where they were allowed to recover before being bagged and shipped to their new mud dam home (and no, we didn’t lose a single fish from the near 200 that were bagged and tagged). We hope that the point is made. Oxygen (aeration) is essential! You cannot overdo it – and if there is too little of it the chances are that it will be too late.