Duck prints on the sand, dead fish in the water

Yesterday I took a bike ride around the neighborhood, and up to the Texa-Tonka park. I had never been there before, but quickly found Victoria Lake. I was enjoying watching the ducks and swans while walking along  the beach filled with duck prints. That’s when I noticed the lake was filled with dead fish. First I saw one, then a few more, then the jackpot. There must be hundreds of thousands of dead fish in this little lake. I don’t know anything about fish, but I’m curious to know what kind of fish they are, and why there would be so many dead fish in the lake. More detailed pictures are below.