Joe,
Adding fertilizer enriches the nutrient component of the pond/lake (phosphorous and nitrogen). These nutrients promote the growth of microscopic alga (plural for algae) which are called phytoplankton. When the phytoplankton grow and reproduce, the green tinge to the water is often called "the bloom". This microscopic plant material is then fed on my microscopic animals called zooplankton (rotifers, cladocerans, copepods, etc.). Then you begin moving right up the food chain. A properly fertilized pond can increase the carrying capacity of the pond (the number of fish) and cause faster and greater growth rates.
The rate of application varies. If you're interested in fertilizing your pond, I would recommend consulting with a biologist/consultant in your area. I'm in an agricultural community so I don't have to do much fertilizing. The run-off from the crop fields and the overall good fertility of our soils doesn't warrant extra fertilization.....at least where I'm located. This practice is definitely something that can help a good pond become a great pond. Good luck.
