I always thought it had to be in the tank for the actual birth -- thinking about the game I'm playing (with the fertility plant, too), I usually get two babies if the greenfin spot is not in the tank (even if it was when I did the mating), and three if it is in the tank (even if it wasn't when the fishies got pregnant).

I'd been moving the greenfin spot so I was only getting 2 babies instead of 3 so the tank didn't get full -- I hope that's how this works, otherwise my strategy is lame

-- jen