If they die quickly but with smiles on their faces, you haven't quite gotten all the alcohol out of the water yet. These are not harmful to either fish, pets or people. If you do want to keep fish in this pond, keep it to just 3 or 4 small goldfish. Do not feed them, as they will eat bugs and larvae as well as keep the sides of the barrel algae trimmed short. Over the last few months there have been reports of oil in ponds from older, submersible water pumps.
Some pumps contain oil.
When the seal on this type of pump fails, you get oil in your pond. The first thing to do is not to panic. Shut off and remove the pump, if possible. If not, then isolate the source of oil in any way possible, within the limits of your plumbing scheme. Next remove the oil from the pond.