Sometimes things just don't work out. I took some great cod, tried to sear it and ended up with a pile of flakes and all the sear on the pan. It didn't taste bad (some glorious bits of crunch but it mostly wasn't the fish I was looking for.)
Looks like Reddit (!) has the answer: https://reddit.com/r/AskCulinary/comments/1v12zx/how_can_i_stop_my_panfried_cod_fillet/. Well, several answers. I might have been overcooking it, putting a bit of flour might have been helpful, or just cooking cod a different way might be easier.