If my memory serves me correctly, anoxia and elevated levels of CO2 dissolved in the Earth's oceans is suspected to be a cause of the Permian extinction 250 Myr ago, the most severe mass extinction in the history of the world. 90% of all species perished, and the marine life was hit particularly hard. As I recall, land animals survived better because their lungs allowed them to expel the CO2 more efficiently. But it's been ... ouch! 10 years since I took Paleontology, so the picture may have changed by now.

In any case, I agree, the fish would not survive long.