Who said fish "need" oxygen?...Oh yeah, it was you
I said their water needs to be airated. Lots and lots of living things...even humans...can live without oxygen at "normal" levels.
What about them crab thingy's at the bottom of the ocean where the hot gases and volcano thingy's are?
If one could remove the valcano thingy, then one would remove the "air" and life would not be...But the valcano thingy puts air where air should not be. Of course the volanco thingy also puts heat where heat should not be....Oops, we have heat, air and water = life!
Air does not need to be oxygen. Air is more a generic word for whatever gas is used by whatever life as a catylist.