I would agree with Bill, I think there would be some weak stratification in a 6 foot deep pond which could lead to anaerobic conditions without aeration. I know from some of your other posts you mentioned that these are lowland ponds you are dealing with so I am guessing the wind isn't really going to cause realiable turn over of the water.

The only real way to know is to do some water testing. I finally bought myself a D.O. meter because it is the only way to really know what is going on. I had the same kinds of questions and I couldn't ever get a clear answer because every situation is different. The D.O. meters are expensive but they do provide the information needed to answer your question. If you were able to monitor the D.O levels in your pond at various depths throughout the summer you could make an educated decision on whether or not it was really necessary.

Another more simple way is to just check temperature at two foot increments until you get to the bottom and try and make some decisions based on temperature differences. If the difference is high, you have stratification.

Obviously you'd have to turn off your aeration after ice out next year to test any of this.

Every pond is a bit different, so the water clarity, nutrients, vegetation at various depths, etc. all play a part.

If you do decide to get a D.O. meter, I bought a Hach and I highly recommend them. They are very easy to use, and also offer various probes that work with the unit, so that you can check PH and MV as well.

Mike