Outdoor ponds often feature koi (Cyprinus carpio), a larger relative of the goldfish. Unfortunately, outdoor ponds also often feature algae, and no algae-eating fish can do all of your pond maintenance for you. You have to perform water changes and use a pond filter to control phosphates and nitrogen compounds. At best, algae-eating fish can help you keep on top of algae. And even then, many species promoted for their ability to eat algae or clean a pond make questionable additions to a koi pond. A few make decent algae eaters, but ultimately, it is your responsibility to keep a koi pond clean, not a fish's.
Pond and Sucker Care
Keep in mind that if you do add a sucker fish to help control algae in your koi pond, it will need additional food. Most pet shops sell prepared food for algae eaters. These usually consist of sinking wafers or pellets, rich in vegetables. Also, sucker fish only eat algae on surfaces, and will not have any effect on free-floating algae, sometimes called "pea-soup" water. Additionally, most sucker fish will not eat fish waste or debris, so you will still have to keep up on water changes and other maintenance to keep the pond healthy.
Pleco (Hypostomus plecostomus) often get sold for their ability to eat algae. They can cause problems in fish tanks, since pet shops often sell them at several inches in length to unsuspecting hobbyists, who don't realize these catfish can grow to several feet long. While they do eat algae and usually won't attack koi, they make a poor addition to a koi pond. First, plecos come from tropical waters. Most cannot survive in the colder temperature of a koi pond. Additionally, tropical fish like plecos and temperate fish come from very different waters in the wild. This gives them resistance to diseases that can kill each other. So plecos may carry disease that can harm koi and vice versa. Some koi hobbyists have reported success with plecos, but the proposition of using one as an algae eater is a gamble at best.
The Florida flagfish (Jordanella floridae) is one of the few native U.S. fish that shows up in the aquarium trade. It can tolerate the cold well enough and can survive overwinter in koi ponds. These fish also nibble on algae. In particular, they eat hair algae, which most algae-eaters ignore. However, they have some compatibility issues with koi. Flagfish may get aggressive sometimes and may fight with smaller koi. Additionally, koi can get substantially bigger than flagfish, and adult koi may snack on them.
Chinese Sailfin Sucker
The Chinese sailfin sucker (Myxocyprinus asiaticus), also called the high fin shark and banded shark, is probably the best choice for an algae-eater in a koi pond. These species come from temperate regions in China. In fact, this species is critically endangered in the wild, but fish farms do legally produce them for the aquarium/pond trade. They eat both small animals like crustaceans and browse on algae. However, they only work in the largest of koi ponds, since these fish can grow to an adult size of 3 feet.