Haha I'm not sure! You'll have to research it. Glass is classic but there are some plastic ones that may work. It will probably be difficult to figure it out. O.o
I have 300 watts of heating on my 90 gallon tank, to keep it at 75F. Room temperature of the tank is 65F. This is considered the bare minimum for a 90 gallon. So, imagine for 240 gallons, at 25F up to 75F. That's a lot of heating.
I still don't know about putting one in with goldfish. It's a pond so it will be a lot harder to watch for agression.
EDIT: Look up pond heaters. I just remembered those exist. Will be way easier than trying to buy 40 aquarium heaters. :p