The algae block drainage ducts and deoxygenate the water, killing the fish.
From the newscientist.com
The result is a blooming of blue-green algae and rafts of noxious water hyacinth that complicate navigation, deoxygenate the water and kill fish.
From the economist.com
Caesium is so reactive with oxygen that it is used as a getter in vacuum tubes, and solutions of potassium and sodium, so called NaK are used to deoxygenate and dehydrate some organic solvents.