“A low dissolved oxygen event” is by far the leading cause of mass fish deaths

"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