bz, With a 30 ft opening your o2 level is fine.When the opening is only 6 inches when it is colder thats fine too as you should have an area around the hole that has opague ice. The additional ice cover provides great insulation against the cold winter temps. Two ft thick ice with air temps at 20 below zero still provides a water temp of 33F just under the ice and warmer as it gets deeper, Dont provide too much of an escape for those temps.Winter is always the tough season to get through for most fish regardless.Even slush is better than snow. Your "demand" for DO is very low and your "supply" is not as low as you may think.Theo and others are on target for your situation,Ill state it again that midwest and northern winter ponds are more likely to be over circulated (not good) than they are under aerated. Keep everyone posted as its bound to turn to winter up there someday.
