Question

describe how animal can act as natural indicators for the acidity of water in lakes and river

Answer 1

Too acid or alkali and the fish die. 
Too acid or alkali and the insects die. 
Too acid or alkali and the plants along the shore die. 
Too acid or alkali and animals will not drink it.

Answer 2

Fishes don't survive in acidic water.
If you spot a water body that contains a large number of dead fishes then this means that the water is acidic.