Question

a boy maintain an aquarium on a fool day he put a small heater in after some hour he found that all the fish is dead what is the reason for it

Answer 1

we know that water has oxygen dissolved in it, that's how fishes breathe by taking in the dissolved oxygen from water.. hot water contains lesser amount of oxygen as compared to cold water. So when the boy installed the heater inside the fish tank it resulted in increase in the temperature hence reducing the oxygen content in the water. Which is why fishes didn't get enough oxygen and died

Hope this helps...