Heated outdoor pond?

[HarlanAshmore]

id put a bunch of pipes on ur roof paint em black and get a 4000gph pump on th end of it

 

[Dr Joe]

You need way slower water flow for that. No time for heat transfer.

 

[sp33dstix]

There are other threads already here. A little search will let you see some options. A convential heater method will be tough for 1600 gals. Look at some of the solar DIY.

I am not sure where you live but if you live in a place that stays fairly sunny check out making a Solar Trough. Here is a cool link I found dealing with that.
http://www.youtube.com/watch?v=kXXOwfZA2Rk
You would need a pump that pumps water from your heater and through the trough. My gf's dad uses one of these in the summer for his pool... Free heating!!!