Oh Oh, need help!

I have a large pond (3500 gallons) that has been home to a dozen koi for the last 8 years. I closed it up at the end of October (Massachusetts) and for the last two days the fish have been at the top of the water gasping for air. I have never run into this problem before and was hoping someone here might have a reason for it and a solution to the problem.

I have a deicer running 24/7 which leaves a good portion of the surface open. (No other pumps are running) The last few weeks have been brutally cold (below zero at times) and last few days have been in the high 20's.


Any suggestions why the fish would be doing this?