It's a fish that doesn't move or function and it floats. They are poisonous so don't touch them or you'll die. If it's decaying that means that the fish scales are falling off and being absorbed by the water which would make the water toxic.
A decaying dead body
They are scavengers and will eat the decaying/dead plants, dead animals (such as a dead fish) and algae. They keep the water nice and clean and the water quality up.
Only fungi feed on dead or decaying matter.
It converts dead and decaying matter to glucose , energy for the saprophyte or fungi
Dead or decaying matter in nature becomes a food source. Nutrients enter the soil and insects and other animals invade the dead or decaying matter for food.
Depending on the species of crab, its food source will vary. Crabs eat algae, water plants, small fish, other crabs, shell fish, and some will eat dead fish or decaying vegetation.
The decaying flesh of dead animals.
Decomposers feed on dead or decaying matter.
Oomycetes
dead and decaying organic matter
A large quantity of dead fish in one single body of water can be caused by many different things. Pollution, temperature change, and bacteria in the water are all common causes for a large number of fish in the same body of water. If the dead fish are kept in the body of water instead of being removed, this could create more of an issue as the decaying fish bodies may cause disease to spread to the healthy fish.
The decaying leaves created a musty smell in the forest.