Well, it is but there are a few quid pro quibus (quos?). There are some practical issues with it for most people, though, and those issues must be addressed before it can be used or even collected. That being said, there are some parameters in natural seawater that aren't as ideal. The first thing that comes to mind is alkalinity. Natural seawater alkalinity is at ~7dKH (2.5 meq/L). At one point in time, it was higher, but has declined in recent centuries, especially recent decades. That's another issue altogether I won't discuss here, though. The other practical issue is salinity. Depending on where it is collected, it may be different than what is ideal for your tank and may also need to be adjusted.
