I feed it to my snails on a regular basis, and the loaches, betta and the shrimp will all pick at it a little bit. I'm sure that all fish need a little bit of greens in their diet, but we can't offer the selection of water plants their used to. (I for one can't offer any or the snails would eat it.)
A good trick is to slice the cucumber into thick slices, lay them flat on a cookie sheet, and freeze them, then freezer bag them. They'll stay separated, keeps for about two months, and will be soft enough for the fish to eat without having to blanch it first.