Okay - plankton is low density when clarity is 13ft+, although at this clarity there is still adequate plankton to grow some fry. I would not fertilize unless later you want to fight more algae and weeds that will grow as nutrients accumulate to make pond more eutrophic. Once nutrients are in and accumulate they are very difficult to remove. IMO appreciate your clear water as long as you have it. Feed some good quality fish pellets if you want to grow more fish rather than fertilize, the nutrients will eventually accumulate to eutrophy the pond.
