Question

fishes survive in the water even if the temperature is very low than 0°​

Answer 1

this is due to tendency of water . water attains its max. density at 4° in case if the temp. decreases below 4° there is no change in state of water . the water on the surface of water body freezes as it is in direct exposure to atmosphere but the water below it remains in liquid state as the surface above has frozen n it is a constant temp.
hope it helps