pond fish dying in winter in virginia

Question
have lost 1 goldfish and 3 koi within one week - they were at least 5-6 years old and have never lost any fish in winter - weather in virginia has been ridiculous this winter - one week 60 degrees and the next 20 degrees - any info would be helpful - by pond is 12X24 feet and approximately 27 inches deep, pump skimmer, waterfall and uv light are all running

Answer
Hi Brenda,
Thanx for your question.  Did you pond freeze over at any time completely?  If the pond froze over even if just a day or two, oxygen is seriously depleted in the water and will cause fish kill.  The bigger the fish the quicker the kill because they use more oxygen.  The only other thing I can think of is temperature extremes but the water doesn't usually respond that quickly to temperature changes.  Did you examine the fish carcasses for disease?  They could have died of a virus or bacteria while in a weakened state.  Fish go into a sort of semi-stasis where they slow down all of their body processes when the water temperature gets very cold.  I hope this helps.
Tom