system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does surgery on fish work and how do they provide oxygen to them while anaesthetised?

I’m assuming it’s always done above water.
gpt
Fish require a lot less oxygen than humans, or even other mammals. So you don't have to worry *as much* about them getting enough.

To do surgery, most of the time they're anaesthetized using CO2 and chilling them way down. Since they're cold blooded, they can tolerate lower body temperatures better than mammals. They mostly just slow down. Also, although they absolutely do feel pain, their pain response is much different than ours and their capacity to experience pain is different, so pain management is less of an issue. So, again, often anaesthesia is just CO2 and cold.

Once they're chilly and knocked out (or as knocked out as fish ever get) they can be removed from the water and put on a table for surgery. A tube is put into their mouth that slowly feeds cool water through their gills. Honestly, as long as their gills stay wet so they don't get damaged, especially with the really slow, cold metabolism, they can get enough oxygen from the air to stay alive. So the water is really just their to keep their gills wet.