Plants and fish are essential to your pond's natural eco-system. They help maintain an equal balance of oxygen and pond health.
THE PLANTS...
Algae is a plant. As such, it requires nutrition to grow. When you introduce desirable aquatic plants into your water garden, they compete with the algae for the available nutrition. And if there are enough plants, they'll naturally minimize the available nutrition and in the process, they'll control the algae for you... NATURALLY!
The FISH...
The fish play a role, too, by eating the plants, including the algae, which helps to further keep the algae in check. Fish such as Koi over 10" in length, when not overfed with commercial fish food, will effectively graze on the algae in your pond.