Read up at this site in the "Articles" section on Cycling a tank. Getting sturdy fish wasn't a good idea until you understand cycling. What kinda fish by the way. The other will likely die also.
What sized tank? What kinda substrate? How much Rock? Were'd you get the rock from? Powerheads? Salinity reading? Need to know a lot more to help you, but first and foremost, you've gotta read up on cycling a SW
fish tank to understasnd what the ammonia and nitrate is all about - and what it does to fish.