Sorry, didn't mean to side track this so much from the pump part of the discussion... Yes, they are sold to feed other fish like oscars. Since they are narrower than a similar length goldfish they are used to feed fish that require live food and aren't ready for the small goldfish that are also sold as feeders. If you already have fish you like in your pond and they are settled in, then it would make sense to quarantine them (or any fish you get) for a few weeks. You could probably keep a dozen in a five gallon bucket in the shade for a week or so to make sure they didn't all come down with ick or other nasties. Just like feeder goldfish they are kept in large numbers in tanks so are probably more likely to have issues than other fish.
Mitch, I've never had a population explosion like that, but I have a smaller pond and don't have lots of shallows or other places for fry to hide which might be what kept the population from growing. I have certainly heard people complaining about mosquito fish getting out of control, which is one of the reasons I avoided them and went with the minnows. If your goldfish are big enough to avoid it, adding a couple large blue gill might start to put the bite on the boom in Rosies and they are pretty easy to sex so you could make sure you are putting in all males or all females if you wanted to keep them from breeding.
