The pondkeeper should make sure that all decaying plant matter is removed, and test the pond for levels of ammonia, nitrites and nitrates. These dissolved nutrients, nitrAtes, are utilized by the algae and your other plants to feed on and reproduce. Most sources recommend that you fertilize once every month or more during spring and summer. If the plant hasn't been repotted in new soil then fertilize an additional time. Additionally, overfeeding your plants will encourage foliage growth instead of blooms.
Many fish foods contain some phosphates and can increase the amount of nutrients in the water.
Feed your fish only as much as they will eat in five minutes. Floating food is a good idea because if left uneaten the food can be netted out of the pond. Another source of nutrients that will feed algae is ground water and rain water. Ground water carries with it anything it picks up. If you have a problem with ground water entering your pond, a berm or a French drain to divert the water into your own dirt garden may be advised.