They should have to come to the surface to breath, whatever the little guys
are.
I remember Brett in Texas saying he used to pour oil on top of his water to
kill off a type of beetle that was chowing down on his infant fish.
Probably you can use something that is cheap, maybe corn oil? When you suspect
they are all dead then overflow the pond and get rid of the oil. It is 100
gallons I think so it should be doable... If you try this let us know how it
works for you.

good luck!

kathy :-)
algae primer
http://hometown.aol.com/ka30p/myhomepage/garden.html