I use trickle filters on all my large tanks and it is not unusual for eggs
to be transported from the tank through the overflow, pass the prefilter and
down into the sump where they hatch and you can see the fry swimming around
in the sump. I guess the next step is that they could be suck up through the
pump back into the aquarium again?
Personally, I hate recirculating systems that are hooked up together because
if you get a problem in one tank, you end up with it in all of them.
Adrian.
Adrian R. Tappin
"Home of the Rainbowfish"
http://www.ecn.net.au/~atappin/home.htm