well thats a sad story, but welcome to AA and to the great hobby of aquariums!
its absolutely great youve read up on this and learned about cycling, as yes it can kill fish, and quick.
i suggest you read up on fishless cycling before starting up that 55
onto that subject, a 55 is a good size for a start SW
tank (the larger the better as parameter changes dont happen as rapidly or quickly as in smaller tanks)
begin to read up on starting a SW
tank as it has some specifics youll need to know before starting
as you come across questions ask them here! this place is a wealth and everyone is friendly here