how old is your baby? If over 6m, I would use a little milk from the fridge, and once the fish is cooked it will flake away very easily. I would mix with the potato and a wee bit of milk then mash to smooth with a fork, if you need it to be that smooth for a young baby.
I give DS dover sole, which flakes and mashes very easily. I bake it in the oven then flake it in with carrot or sweet potato. Once it's a bit cool, I express in some EBM then whizz it with the blender thing.