"By far the leading cause of mass fish deaths is lack of oxygen, and often oxygen depletion"
https://blog.nwf.org/2014/08/algal-blooms-and-mass-fish-deaths/
"Thousands of fish washed ashore along the Texas Gulf Coast due to “a low dissolved oxygen event” in warm water, US officials have said."
https://www.aljazeera.com/news/2023/6/12/mass-fish-deaths-on-texas-gulf-coast-due-to-low-oxygen
"Many of Florida’s fishes are dying for the same reasons as in other parts of the country: lack of dissolved oxygen"
https://www.themeateater.com/conservation/wildlife-management/12-states-suffering-massive-fish-die-offs
"The most common cause of a massive fish die-off in a pond is a lack of oxygen. The result is almost always a shoreline of dead fish."
https://www.news-journal.com/features/lifestyle/davis-lack-of-oxygen-in-ponds-can-result-in-fish-die-off/article_c4914f86-df66-11eb-b3b0-6b95e966d991.html