Very sorry to hear of your loss. I had a similar situation where the fish were coming to the top mouths open screaming for air. My water fall pump had failed and when the sun went down the oxygen level dropped. I had to put an air pump in my filter and run it around the clock for several days to get the pond back in balance.

I hope your loss is limited to the fish already affected. I can imagine how you feel because every fish has a name and personality. I know my koi did anyway. I never had sturgeon but had dwarf catfish that helped keep the bottom clean.

Good luck.