At the very least the water would have to go thru a very fine filter first or be stored in a dark container for a few weeks to be sure all active pathogens have been killed. It is too easy to use artificial seawater as it is rich in calcium, which you will want, and it is free of contaminants and pathogens. This is a small closed system and it take very little to upset such a system. In public aquariums that handle millions of gallons, about half of them use filtered seawater and about half use artificial. Where you live you have a good chance, especially if collected a few miles off shore of having suitable water. But that seems like a pain to me.
, I would go for a small coral garden and concentrate on smaller fish. Forget about tangs and big angel fish, think gobies, wrasses, fire fish, etc. there are many, many to choose from.