I had the same questions of two these guys before me!
For fist I thoght and I put "sanduiche com peixi"... but the answer controller said me "Error, is only: sanduiche de peixe"...
Why? Can someone explain it, please?
To my thinking, um sanduiche com peixe should be two slices of bread with a fish fillet in the middle, but um sanduiche de peixe is a sandwich made of fish, so two fillets and no bread at all and maybe some aioli and a wedge of lemon inside.