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Just my two cents worth:
Many years ago, I kept saltwater tanks that contained local tide pool creatures, which I collected with a permit from my state. Our public aquarium at Scripps Institute of Oceanography allowed residents with personal aquariums to fill up plastic Jerry cans with the natural filtered sea water from their tanks. This was had already been through their aquariums' filtration systems, and had all the natural trace elements found in the ocean where my creatures had come from. It beat using Instant Ocean, any day. Much more natural and the kind of water the tide poolers were already accustomed to. In other words, it matched their natural environment.
This may or may not work with tropical coral reef creatures, but for my purposes, it worked out well.
Just a thought. You would have to take into consideration the parameters of the waters in which your marine creatures lived in nature and try to match those as closely as possible.
I'm just saying that under certain circumstances, using natural saltwater can be beneficial in a marine aquarium.
20 gal. high: planted; 5 white cloud minnows, 4 golden White Clouds, several RCS, 2 blue shrimp, 5 Amano shrimp, several snails; Azoo air. 65 gal: planted; 6 rosy barbs, 6 yellow glofish, 3 red glofish, 3 zebra danios, 5 white cloud minnows, 3 dojo loaches, 6 crimson spot rainbow fish, 12 large Amano Shrimp, several snails; AC110.