Most crabs are opportunistic omnivores, and given the chance, this crab WILL eat you fish. No, he is not a vegetarian, and do not think that he is. He NEEDS brackish water, and access to land.
I cannot stress enough how important researching new additions to your tank before you get them is. If you refuse to do this and keep buying on impulse every time you see something you are, to be frank, a bad fish/crab keeper.
If you really do love this crab as much as you say, you should return him to the store or set up a tank just for him.