Carp fish in outdoor fish pond

Question
My daughter recently moved into her first property including an outdoor fish pond containing 8 carp and two gold fish.Pond completly frozen over for 4 weeks.Made hole in one side of ice but was to thick to make more than small hole at side.Poured boiling water over ice to try and melt it .Did not work.Has also snowed so for two weeks and ice has been snow covered though maintaining hole in the side.Keeping it open.Over past two days ice has been gradualy melting.Also rainfall yesterday dispensing all the snow.Early this morning I found all the fish on their sides floating on top of pond .All 8 carp have died.The two goldfish were barely alive however putting some clearwater solution to tap water in a small tank they revived .Could you please tell me in your opinion what has caused this to happen.The pond is located under trees.Is over 20m.Large stone design raised pond.There are plants in plastic buckets in the bottom of the pond.Could these have contributed to the problem .Am absolutely devasted losing these lovely fish and apreciate please any help/advice .Kindest Regards.Susanne.X

Answer
Hello Susanne!
Our newsletter this week dealt with just this problem. There are two things that are big pond fish killers in the Winter:
a) lack of oxygen
b) stratification
In a winter pond oxygen can become an emergency really quickly, especially with the presence of larger Koi. It is best to use an air pump in these conditions. Considering the value of large Koi it is a rather small investment that can be re-used every winter.
Once a pond is covered with snow, the situation can be become even more critical, because if any of the submerged plants are still green they will take oxygen out of the pond (the plants are called Oxygenators...but that only works if there is light, without the process reverses). It is important to sweep whatever snow you can safely reach from stable ground off the pond (an air pump makes this unnecessary).
Stratification happens when the sun suddenly comes out for an extended time and starts warming the top layer of the pond water. The pond can actually "flip". In this process the warmer water (which is denser and consequently heavier) suddenly sinks to the bottom and the colder bottom water rises quickly to the top. Koi are extremely sensitive to sudden temperature changes and this process can kill them within minutes. This has happened to me once and it was heart breaking. It happens very quickly and there is nothing that can be done at that point.
An air pump helps avoid this problem, also, because it does help to mix the water somewhat on an ongoing basis.
If your daughter would like further Pond care information, we have several free eBooks on our website and there is the free newsletter that she can sign up for. It usually covers seasonal issues.
Brigitte
www.pondmarket.com