I don't know about the soils and subsoils exposed in your pond basin, nor the content of the incoming water.
However, the formula for most ponds is that fertility increases going forward in time!
Further, that minnow "chow" seems expensive compared to firing up the natural productivity of your 0.5 acre pond. Maybe read some of the pond fertilization threads, and see if any show a successful methodology that matches your geographic area?
