That seems amazing, when you think about all the goldfish that are bought and then suffocated in dirty fishbowls, but one little goldfish in a big rain barrel--that would be goldfish heaven!
My wife always had a few feeder fish in the horse troughs to help eat larvae. Horses never seemed to mind and honestly I can't say it really made a difference....but then again I also had a few bat houses up and that REALLY made a difference. Maybe you should look into taking that approach as well attack the problem from multiple angles.
I second that, Jared77. Bats are very helpful and fun to watch on a summer evening. We have several in our area but I don't know where they hang out during the day. We've talked about putting up a bat house to help lure more in.