Question

why is phosphate free detergents helpful in saving fishes

Answer 1

because as it is the main cause of eutrophication its reduction helps in reducing the eutrophication rate which helps fishes to use the dissolved oxygen in a correct rate.

Answer 2

Phosphate detergents form algae and other harmful layers on the water which can cause the shortage of oxygen supply to the fishes. Therefore phosphate free detergents are helpful in saving fishes by helping them take adequate amount of oxygen.. Hope it helps......:-)