DIfferent manufacturers have different ideas of what it should be. The difference is that while we try to recreate to some extent the natural occurances in the ocean, because we're dealing with limited volume setups we don't have the benefit of millions upon millions of gallons of water to back that up. Our inhabitants consume the elements within the water, right? Corals for instance suck up alk
(and other trace elements) to grow. In the ocean, as a coral consumes some of these things, the vastness of the ocean doesn't even notice because the amount consumed vs. the amount available is negligable at best. In our systems, if a coral consumes the elements, since we have limited systems, the slight consumption can cause a noticable decrease in the overall system in terms of quantity available of the elements.
I hope that made sense.?.?
Because of this, different mixes attempt to combat different things. Certain mixes are geared more towards people with Reef systems, and in these mixes the concentration of those elements needed by a reef are typically higher than what you would have in the ocean, to try to combat against levels getting too low as the elements are consumed.
Once we get that understood, we see that different setups have different requirements. For instance, a FO
tank doesn't need the higher salinity, and having it slightly lower than the natural occurance in the ocean isn't unhealthy for your fish, but does help to ward off some diseases and such, so it has it's advantages. That's just 1 example.
Did any of that make sense?