Hi big ron, it is true that the pond should not be sealed air tight. Gas exchange is important, especially if you have fish. I use a net with 3/8" opening for my 16'x5' pond. It takes care of most leaves and debris. Not sure which zone you are in but if you are in the colder climate, instead of using heater, use the bubblers. You can even turn off the pump since the 2 air stones will keep a nice hole open. I use 2 air stones and the cheap fish tank air pump I bought from the Wally World for $20. I've used it 3 years. So far, fingers crossed - I have no fish loss. HTH.
Kuan