Are you using the boxed water or actually getting it from the ocean?
First and foremost, I would say natural seawater is just about ALWAYS better, unless you are getting it from a boat launch.
Synthetic salt mixes are way out of proportion in terms of there concentrations of elements than NSW, and many animals have great difficulty adapting. This is one of the reasons echinoderms like seastars have such difficulty in adapting.
So in short I would ALWAYS use it if I had a ready means...I think the boxed stuff is relatively less ideal and is cost prohibitive for me, however.