The "fatezero" tag is for the Fate/Zero series. Fate/stay night is already rendered as fate-stay-night and I agree that the slash should be replaced by a dash instead of just mushing the two words together.
Did a rename for this. I decided not to put up a synonym for now because, 'sfar as I can tell, the dash-less version was the result of the original input and not something we expect to run into. But, if we figure a synonym is really necessary or if we just want it, no harm in setting that kind of thing up.