In episode 40 you see Sanji fight Kuroobi the fish man underwater. Sanji defeats Kuroobi by blowing air directly into his gills and suffocating him. However fish actually breathe by taking water in through their mouths and expelling it through their gills. So in order for Sanji’s plan to work he would have had to blow the air into his mouth and thusly force it through Kuroobi’s gills. Air being blown directly into a fish’s gills would probably still not be very healthy for it, but I think it would just be mildly irritating and the air would most likely be expelled almost immediately as the fish continues to breathe. I highly doubt that it would have caused enough damage for Sanji to gain the upper hand.
I am an avid aquarist who has kept fish since I was 11, so when I saw this scene I immediately knew that something was wrong. I did some research to make absolutely sure that I was right, however if you do see a flaw in my logic or facts please let me know!
I know that this show has fantasy elements and shouldn’t always be taken too seriously, but I just thought I should let you guys know.