Alright, I felt like spriting something, so now we have this ungodly mess with what is probably an incoherent mess of a description.
This freshwater fish takes small bites from plants without killing them and is born from an egg sac typically containing around a dozen offspring. However, the egg sac is far too big for the fish and also keeps vital organs in place, so it almost always dies after releasing it. They tend to form small schools of two to five fish for safety, and flee from their natural predators, but fail to group creatures under the 'natural predator' category, making them an easy meal. If an adult fish is eaten or starves to death, it prematurely releases a shriveled, deformed egg sac that doesn't contain as many young.
As for their nutritional properties, the newborns are very tasty, and can create carnivorous tendencies in creatures who wouldn't otherwise be interested in meat or reduce cases of maladaptive learning in natural carnivores, but it would take many of them for something resembling a decent meal due to their small size. The juveniles are little more than a generic snack, being nutritious, but not having any special properties. The adults, on the other hand, are a filling meal, and take a few bites to polish off. The bones are soft enough to eat, but still provide a satisfying crunch. The egg sacs don't taste good, and creatures may be discouraged from eating eggs in general by the awful taste.
The 'sleepy predator' phenomenon in areas where Midnight Trout are present is caused by the prominent presence of sleep toxin in the flesh of the adult, although no major harm should come from eating it. The rotten fish is said to taste like lime-flavored Jello, but attempting to prove it is not advised unless one plans on dealing with severe belladonna poisoning. Egg sac waste isn't as toxic, and can be safely eaten by scavengers.
Very rarely, a dead adult fish that hasn't been partially eaten can reanimate, becoming a zombified beast. It secretes belladonna into the water, causing the living fish around it to sicken, die, and, in the case of adults, turn into more zombie fish. The resulting egg sacs won't hatch until the horde leaves, which they will unless it's a small pond they're in, in which case they'll linger until they go back to being normal, non-zombie dead fish.