bc - you'd better do a little more research and avoid granular ag fertilizers.
For small ponds, a quality water-soluble fertilizer with a relatively high phosphorus:nitrogen ratio(ex: 12-61-0 ~ N-P-K)will usually suffice. The use-rate will depend on the % of P and your pond's unique water chemistry. Acidic/"soft" waters will generally not respond to fertilizers. So, if in question, have your water tested for pH and hardness and, if necessary, treat with ag lime well ahead of the spring-time rise in water-temps.
Liquid fertilizers (10-34-0 or 11-37-0) may also be used, provided you dilute them AT LEAST 1:10 with water prior to applications. Otherwise, the high densities of liquid fertilizers will tend to sink them to the bottom - where they'll primarily feed macrophyte plants and/or forms of filamentous algae rather than phytoplankton.
