I can totally relate to fish dying and not being able to keep them alive, and my tank has been running for over a year with no obvious reasons as to why the fish breathe hard and die hours later.
However, eveyone will try to help as much as possible as long as you provide your water parameters and so forth. I am still working out my problems with help from this board and it has been almost a month now and for us impatient people, that's like a year!
Based on what you have posted, I'd advise to leave your tank alone for a while and not add any more fish until you figure out what is going on.
How long has your tank been running? (I don't know if you posted that already.) And as we found out, freshwater aquarium decor is not for saltwater. (You mention a castle.) We tried that when we first started out, after all, the tank in Finding Nemo had lots of decorations, right?? BUt it really just isn't suitable for SW.
Please get back to everyone here, and let us know what is going on.....