I am a fisheries biologist for a private pond and lake stocking company in MO. In highly productive mid-west soils we rarely have a problem with not having enough nutrient load to sustain high levels of growth. We typically have the opposite problem with hyper-eutrophic systems where dissolved oxygen will likely end up being the limiting factor of productivity. Aquatic systems are usually phosphorus limited and occasionally nitrogen limited, never seen a case of potassium limitation in a pond/lake.

In a commercial production setting we do fertilize our ponds with 16:32:0 to jump start/super charge our phytoplankton and zooplankton production. Most species we raise readily accept a commercial diet and we supplement their diets with that. Fertilization isn't necessary in a natural setting and can be detrimental to water quality, we regularly run commercial aeration and flush ponds with fresh well water during summer months. A feeder can be a great addition to a farm pond/lake if the landowner is trying to push the growth of their fish.

The most important factor for growing your fish in a farm pond I have found is to have correct habitat for spawning of forage fish and by stocking the forage fish (minnows and panfish) prior to any predatory gamefish. Many people think deeper water is better because it us supplying more water/area for the fish to grow, but actually most fish spawn in shallow flats and shallow water will be the most productive for zooplankton, phytoplankton, and invertebrates. Deep water is great for over-wintering fish and providing additional water volume to reduce oxygen depletion. So having a variety of depths and habitats is important as different species will have different requirements, and each species needs will change with the seasons.

Your clay is a blessing, make sure you get your dam cored and sealed well and will help minimize water losses through dry summer months (I am from central NE originally and know how dry the summers can be). As jpsdad has pointed out, your soil will have ample organic nutrient load for you plankton bloom.