I figured you wouldn't want to eliminate water changed completely.
Anyway, you're probably right about experimenting. You can always get use the Y and ball valve people use when they have sumps. Basically in the output for the pump, you attach a Y. On one Side of the Y you'd have the coil, and on the other side you'd have a ball valve that leads back to the tank (or sump, in the case of a sump). You then simply adjust the ball valve more open or more closed to get the flow you want through your coil.
Also, how do you plan on getting the DC
washer fluid pumps run on AC
power? I guess you could just buy a 12v adapter and do some simple wiring. Aren't washer fluid pumps loud? Just things to consider. I wonder how much GPH
those pumps are. Also, I doubt they're submersible, so you'll have to do an external mounting. And you'll have to find some piping/tubing that'll mount to the pump nicely. (I'd salvage some washer fluid piping from a car). If this works out well, I might have to make a trip to the junkyard. :p