Welcome Jimmy,
Fertilizing is to promote a phytoplankton bloom. They (the critters) are the bottom rung of the food chain essentially. Everything else above it kinda counts on that solid base. Many ponds naturally have plenty of phytoplankton. As long as you don't have turbidity issues, folks often measure the health/density of the bloom via Secchi disk.
Here's a useful post about fertilizing.
Ewest's fertilizer info Here's a post from a guy discussing pond color and folks advising on bloom/fertilizer.
Pond color
