Carolyn reports that when she got a rain barrel to collect water for a vegetable garden, she had to figure out how to prevent mosquitoes from breeding in it. And rather than use chemicals or BT, she invested in some goldfish, figuring they would eat any mosquito larvae:
... This has worked really well. The fish, which were previously designated as food for larger pet fish, have 55 gallons of space, both fresh bugs and dried fish food for dinner, and no predators. It's a bit boring in there, but I'm guessing it's suitable for a fish-style attention span. They've grown appreciably in the past couple of months. I think they're happy. At least they haven't complained. Or died. Yet. ...