Have you ever had the opportunity to travel south to La Jolla and visit the awesome public aquariums at Scripps Institute of Oceanography? I used to live in that area (north San Diego county), and when I had my collector's permit from the state which allowed me to collect tidepool specimens for my coldwater marine tank, I would go down to Scripps with huge plastic jerry cans and fill them with the filtered seawater from the aquarium, which was free to the public for precisely that purpose, then transport it back home and use it for water changes. Never had to add trace elements, as the natural (filtered) seawater had plenty. And since it was the same seawater as the tidepool creatures I was keeping normally lived in, it was so much better than mixing up artificial seawater using Instant Ocean.
Just wondering if you have ever been down there? If not, you should go. It was amazing back then, and probably still is, today.
20 gal. high: planted; 1 zebra danio, 6 glofish, several snails, 2 (visible) RCS; AC50, Azoo air. 65 gal: planted; 4 rosy barbs, 6 glofish, 5 white cloud minnows, 3 zebra danios, 5 dojo loaches, several snails; AC110 x 2.