you have wonderful friends, dan
depends on the rating of the fish tank pump, but i would recommend getting a good, new pump for peace of mind.
such a large volume of water may indeed let you run without a radiator, i'd say give it a try and if it gets too hot, just plug a big radiator in and run that passively. one thing to note about using a fishtank is that you'll have to make sure it stays clean - algae and other gunk that builds up there will find its way into your loop and that causes all sorts of problems. hence you'll need to keep the lid on it all the time, and you'll also have to run a large amount of radiator coolant / corrosion inhibitant to keep things in check.
lastly, if possible i'd recommend putting the tank at a lower
altitude than the system, as pumps are generally designed to work better pumping water up than down. it still might work though, sounds like a case of trial and error to me.