Minecraft 18w08b, Minecraft 18w09a, Minecraft 18w10a, Minecraft 18w10c, Minecraft 18w10d, Minecraft 18w11a, Minecraft 18w14a, Minecraft 18w14b, Minecraft 18w15a, Minecraft 18w16a
1. Place a cod on dry ground and watch it die an agonizing death (you monster!)
2. You'll notice that the fins of the cod show z-fighting (see screenshot)
This neither happens with other dying fish nor with fish swimming in water (including cod).
Some of the new tropical fish variants show this issue too, I don't know their specific names though.