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. The mulm contains nutrients and, when free-floating in the warmer surface water and receiving light, can feed algae. The answer here is to try and keep the bottom of your pond clean, but not sterile. Mulm is beneficial in the pond, but should be controlled to within about 1 inch thick for a healthy pond. Additionally, and in ponds that are in their second year or more, organic debris has had the winter to continue the decay process and the pond contains many nutrients.