Fish - consume algae, aphids, mosquitoes, flies & mosquito larvae.  Goldfish produce carbon dioxide and waste which is high in nitrogen.  Goldfish are very easy to care for in an outdoor pond, as long as their basic requirements are met, these being food and oxygen as well as clean water.  Their waste is broken down by bacteria in the water, which them fertilizes the plants.  The sludge collected off the bottom of the pond at year end can also be used to feed your nearby flower beds.  If you are able to overwinter your fish outdoors, it will be helpful for you to know that fish settle in the deepest area of the pond.  Fish do not digest food in winter, and they require very little oxygen, so it is unnecessary to worry about feeding them in the winter months.  You will need to begin feeding them again in the spring, once the water temperature reaches 10 degrees celcius.  An ideal first food in the spring for fish is wheat germ flakes.  This helps clean out their digestive system.  If you find your fish coming to the surface gasping for air, then you likely have an oxygen problem.  This can be fixed by adding more oxygenating plants and/or aerating the pond with fountains or waterfalls.  You may need to install an aerating nozzle at the pump.  Remember, moving water is aerated water.