Question

why does unturned wastewater kill fish

Answer 1

first of all its untreated ok.
your answer is :
There are several reasons, but out of the primary reasons is that wastewater tends to be extremely high in ammonia. Ammonia is poisonous and is deadly to fish, even in relatively small amount.
Hope it help you!

Answer 2

Explanation:

Water has limited amount of dissolved oxygen. This oxygen is depleted due to abundance of phytoplanktons. The bacteria that decompose phytoplanktons, require large amount of oxygen. They consume dissolved oxygen. The BOD level of water drops below 6 ppm which inhibits the growth of fish and causes excessive fish kill.