Since buying explosives is problematic and can get you on a gov-list just go to a builder supply store and buy some quicklime (calcium oxide). Fill a quart Mason jar about half full along with a rock or whatever for weight. Put the lid on tight then poke a small hole or two in the lid. Now toss the jar into the pond. In just a few minutes after it sinks you will hear a muffled thud and the fish will float to the top. Quicklime when mixed with water forms slacklime or calcium hydroxide. The process is exothermic and the pressure of the jar and the surrounding water will cause an explosion, a very hot explosion.