I dont like the idea to "feed" a plant with a living fish...
Nepenthes don't catch fish on the wild so why do that?
I go with the idea to feed the plants as similar as they do on the wild (I guess some nepenthes will catch frogs? I stick to insects, as kath say "not all animals are the same").
Second, I think is cruel even if the fish is sick... kill it and then feed it to the plant if you like, then will be a diferent situation!