What if?

[addy1]

I have a small pump in the shallow end around 15 feet from where the fish hang out. You are still removing gases that can build up with the open hole. They sometimes wander down to the open area, but not very often. The pond water is 40f.

 

[Gemma]

I thought having the large aerator would keep a decent size hole in the ponds but these last few days a dome of ice formed over both of them, leaving open only the perimeter of the hole. I know that's enough for gases to vent, but with temps dropping some more for the next 3-4 days I'm not so sure that gap will continue to stay open.
I plugged in a heater in the front pond where most of the fish are, I don't have a heater for the other pond, do you think with just the 4 little koi it would be ok if the whole closes for 12 to 24hrs (we can open a whole once a day after work).
How long can fish survive in a pond without a venting hole?

 

[addy1]

I think it will take a while for the gases to build up. With only 4 fish you are probably ok.

 

[Gemma]

Thanks @addy1