It seems very reasonable doesn’t it. We keep seawater aquariums and so why not put [tag-tec]natural seawater[/tag-tec] in them. It saves money too, no more buying those expensive buckets of dry salt mix.
The first consideration is availability. Seawater is heavy, around 10lbs per gallon. If the aquarium is fairly large the amount required will be many gallons, much more if it is an initial fill. Lots of large containers, all suitably safe for seawater, plus a vehicle that can safely transport the bulk and weight. Most aquarists live away from the sea, and the dry salt mix is the obvious way to go.