Question

all fishes in a certain village are dead and the surface of the pond is covered with a green growth what could have caused the death of the fish

Answer 1

This green growth is called eutrophication, it block the sunlight in water bodies due to which plants are unable to do photosynthesis and lack of oxygen leads to the death of fishes.
Hope this will be helpful :)

Answer 2

may be the pH of the pond become less than 7.and it turn acidic which results the death of the fish.
hope this helps you...
plz mark it as brainlist answer.....