the ocean have current every sec to exchange water, while our aquarium only have specialy amount of water.
don't know if u even heard of monterey aquarium. (I think most big aquarium do that too)
they direct their water from the ocean to their whole aquarium. making the ocean the big sump~
you can try to use ocean water, but I will keep my eyes on para and other bad creature come with the water. since the water u get from the ocean will probably have tons of live form inside, if they die off in your tank, then your nitrate will raise like a rocket.
Thanks to AMD, HN1, Zero, partypalooza, thincat, and thominil who helped me in almost every questions. Thanks everyone!!!!!