Question

how can an increase in algal population size kill fish

Answer 1

One of the negative impacts of eutrophication and increased algal growth is a loss of available oxygen, known as anoxia. These anoxic conditions can kill fish and other aquatic organisms such as amphibians. ... It is true algae produce oxygen, but only when there is enough light.

Answer 2

Answer:

Algies grow on water surface takes up all the dissolved oxygen in water which leads to lack of oxygen for fishes and moreover death.

This algal growth is aslo known as Eutrophication.

Hope it helps...