The very best thing you can do for small fish is fertilization by organics in a pond that is freshly filled. Zooplankton can eat it directly and as it decays it will release nutrients for phytoplankton creating a smoother bloom sequence and better water quality than bloom created by inorganics (which blooms can get away from you more easily). Rice bran, cottonseed meal, and alfalfa are all good for this type of fertilization.
I have read that one of the main differences between inorganic (commercial) fertilizers and organic fertilizers is that the organic fertilizers generally release their nutrients much more slowly. I like your advice to therefore use a locally available organic fertilizer.
My forage ponds are going to be located in our tallgrass prairie field that we bail annually. Would native grass be a decent organic fertilizer? If not, should I create a compost pile adjacent to the pond and use some partially decomposed grass as fertilizer? If so, can I just toss in an occasional pitchfork load, or would it be better to fence a small area for concentrated final decomposition in the forage pond?
