if i could, i would use NSW.. but, regardless of how clean looks on shore, i'd still try to get it as far off shore as possible. have a boat?
either way, even from shore, if it's not around any ports, it should be much better than your tap water is.
i wouldnt mix with homemade salt water though. i mean, you'll have to for starters, but i'd just use the nsw from now on.
run it through a poly filter at least. and check the levels on it. ie: ammonia, nitrite and ph. i would assume that you'll detect no amm. or nitr. you might also check the salinity level to see how it compares to what you are using now.
overall, you'll save money by not getting an ro, and you'll save by not needing to buy salt any more
fwiw, i have an uncle in tampa wiht a 250 gallon sw tank. he takes his boat out onto the gulf once a week and fills up a garbage can with water from the gulf, and uses that as his water change/top off water and so on.
he's had his tank up for about 5 years, and has never bought water or salt for it. but he does go out quite a ways for collecting, but that's cuz tampa bay aint the cleanest thing around
