Does your water look like this in summer (plankton bloom)





Triple Super Phosphate
0-46-0 can be purchased at any Co-op or feed store in 50 lb bags. There are other water soluble fertilizers also which are easier to apply. They are the major method of adding P to a pond. NPK fertilizers are sold by numbers with the first being nitrogen (N) the second phosphors (P) and the third potassium (K).

Before you decide on the fertilizer course you need to search hear and learn all about the process and be sure it fits your situation. Many ponds don't need fertilizer and you have to know about the process first.