Tom, I'm really out of my element on this one, so I can't recommend how far off the pond bottom you would want to be with the outer pipe.
The reason I knew about that method of using an outer pipe is because it was discussed on this forum maybe 6-12 months back.
Your posting is fairly new, so as the day goes on, others will give valid input.
By the way, your project sounds great. That's really deep at 38'.
Good luck.
