Most treatments (i.e. poisons) for something we don't know anything about will kill the fish before they will kill what you guess you are maybe aiming at.

If you didn't like those critters which were feeding on the carcass, why in the world didn't you take a cup, jar or anything which would draw water, place it in front of the body and feeding critters and draw as many of them as possible out?

"AH! The house is on fire! Aren't those flames lovely to watch!"

We just saw our dog limping. My wife wanted to give him a pain killer. I preferred taking a look and then getting the thorn-like stick out of his paw.

At the very least you would have removed some of the immediate problem by pulling some water and those things in it. You also would have removed what was feeding the immediate problem and allowing them to rapidly multiply. At best you could put those critters in a bottle and maybe look at them through a cheap hand lens. If you are a student or know someone at a local school, you might have been able to contact a science teacher and ask them for help under a microscope.

If all else fails when you are taking stuff like that out, put them in a small clear bottle or if nothing else water tight is handy, an old pill bottle. If you can strain those creatures out through a brine shrimp net, sieve or or fine cloth, put them in a little clean water. Double the fluid with a little rubbing alcohol and seal the bottle. That will preserve them until you can find a way to magnify them and also borrow a book the on basic use of microscopes (microscopy?). They often show many of the small creatures from aquaria. Books on invertebrates can also be useful.

Sometimes we don't think of what could be fed to our fish. There are several books on foods for fish, often out of print, but available through interlibrary loan. You may be surprised to find the creatures you are concerned about in there.

In what were originally my Daphnia cultures outside, I have found mosquito larvae and eggs (which are hopefully pretty quickly netted out and directed to the appropriate tanks), blood worms, white worms (August on), Dero worms, Ostracods, tiny Ostracods, Cyclops, and more, probably carried in either by egg laying parents or on the whiskers of assorted varmints like raccoons, skunks and 'possums. All of those are good fish foods, though you might not want to leave the tiniest fry with some.

Some of the live foods books would include:

Masters, Charles
Encyclopedia of Live Foods (HC)
ISBN: 0876660936
TFH 1986

Jocher,Willy
Live Foods for the Aquarium and Terrarium.
"McDaphnia" notes that "It's a 1973 TFH republication of a 1966 Studio Vista Ltd. translation of the original 1965 German book.

Needham et al's Laboratory Culture of Invertebrate Animals (Dover about 1962).

Less useful, but handy might be the Baensch Atlas vol. 1.
or the old Wm Innes book - Exotic Aquarium Fishes

I have heard of, but haven't secured,
Live Foods for Aquarium Fishes
(Hardcover)
by John Rundle

It is ok to search for live fish food or live foods for fish on the library computers or on Google.

I might add that I have sometimes found the larvae of predatory insects and fed them to aquarium fish. "If it can fit into the mouth...."

Sometimes, as with voracious and nasty water tigers, I have frozen them before offering them to the fish.

If there is a dead fish in an aquarium, nature will try to clean it up. The result (among other things) could be a bloom of bacteria (white or stinky water), planaria, cyclops, fungus and many other things.

See also:
http://www.livefoodcultures.com/
http://www.lfscultures.com/  (down at the moment)
http://web.cecs.pdx.edu/~davidr/discus/livefoods/cultures.html
http://betta-barracks.ourfamily.com/live.htm
http://fins.actwin.com/mirror/live-food.html
If you aren't in a curious mood that is ok. But get the dead fish out! :)

Over 90% of our "luck" is what we do or don't do.