A boy maintains a aquarium. On a cold day he put a small heater inside theaquarium to make water in it slightly warm. After a few hours, he found all the fish dead. What is the reason for it?

The solution is as follows:

1. Like humans, fishes also need oxygen to breathe and stay alive.

2. Oxygen is more soluble in cold water, than hot water.

3. When he put a small heater in the aquarium, all the oxygen from the water was expelled out and fishes died due to absence of oxygen.