So, I've questioned myself a few times over having made my pond so deep (6 feet, with 4 of those feet being in ground and 2 above). It makes maintenance tricky and reduces viewing as even with the water clear and the sun shining, I can only see down about 3 1/2 to 4 feet max. Plus there's so much plant cover (floaters) and the fish like the deep water, so I often don't see them much aside from feeding time. However, now with the weather getting colder, I'm really glad I did it. Last night the air temp got all the way down to 48 degrees. I just fed my fish and checked the pond water temp - it's at 58. I know the pond hasn't started to warm back up from the night yet, as there's no sun on it just yet, so I can pretty confidently assume that 58 is as low as the pond dropped last night. That's 10 degrees warmer than the air (and still 5 degrees warmer than the air right now - we're up to 53 outside)! I'm thinking that, if I build a PVC and poly cover, I can probably keep my pond above 50 for all but a few weeks of the season. Somewhat OT, but I'm also happy that my pond bottom seems to be staying pretty clean since I cleaned it with my python/garden house hookup. I figured I'd have a bunch of plant debris down there, but I dropped my filter box right down to the bottom (hooked a rope to it so I wouldn't have to go swimming - brrrrr!) two days ago. Pulled it up and checked it today, and it's still nice and clean.