Neat idea, in a messy kinda way.
It might be prudent to know how the dead critter met it's end.
Botulism is a food poisoning. The toxin-producing bacterium, C. botulinum, is common in surface soils of both the terrestrial and the aquatic environment. Toxins are produced only when conditions favor growth and multiplication of the bacterium.
Type C toxin occurs in carcasses of dead invertebrates, in the flesh of birds dying of botulism, and in maggots feeding on toxic carcasses. In a type C botulism outbreak the sequence of events are: aquatic invertebrates ingest the bacterium when feeding and a lowering of water levels or an increase in water temperature causes large numbers of the invertebrates to die. The carcasses of the invertebrates provide a good medium for growth of the bacterium with subsequent production of toxin. Ducks feeding on the toxic dead invertebrates found in the bottom sediments become poisoned and die. Maggots infesting the duck carcasses become toxic and are eaten by ducks and other species of birds, thus increasing bird mortality in an explosive manner.
I'm sure there are other toxins that could be passed to your fish via the maggots. It probably would be a rare thing to lose fish this way, but it's worth considering.
