Mine were 80F top to bottom in the morning and 80 at the bottom and 84 at the top by evening over the weekend with 24 hr/day aeration. Highs were in the upper 80's and nigh time lows were upper 60's.

I have found the the top surface reading can be very tricky. I tend to get the thermometer about 2 inches below the surface and have found the the upper 2 inches can fluctuate by as much as 5 degrees. my pond can be very still with little wind.

Fish on!,
Noel