Bruce something like this in the 1800 watt range would be good investment if this is a longterm project for mutiple year use.
http://www.aquaticeco.com/subcategories/3033/Process-Tech-Heaters-Single-Phase

For fish oxygen demands for emrgency with pure DO. We selected this to sell. We sell a bunch of these for transportation of fish. It is kinda like George system but ready to go. I suggest you purchase form local weldign shop your oxygen cylinder. http://lakework.com/cart/index.php?p=product&id=181&parent=16

Greg Grimes
www.lakework.com