I would suspect that it may also be pesticide runoffs. While the Fertilizer runoff can help get too much algae growing. The Algae would have to die off and become food for oxygen consuming bacteria that would lower the water's Dissolved Oxygen levels. Low DO levels can kill fish.

But a pesticide runoff will kill the fish directly.

Sounds like someone may have had a leak in a big tank or dumped something into the water shed to cause a big fish kill.

This time of the year the farmers may be applying pre emergent pesticides to help their crops get a good start against weeds.

Regards,

Moose1am