I wrote and article for the FNZAS magazine about saltwater collection but here are my tips:
* Always collect water on the incoming tide, it will be cleaner and less likely to contain contaminants and suspended matter.
* Never collect within 3-4 days of rainfall. Rain creates run-off, run-off contains all sorts of pollutants (fertilizer, nitrates and phosphates) from the land, all of which are bad.
* Your location should be in a high flow area, away from any industrialization (pipes, boat ramps etc).
* Never collect from a site that look like its contaminated (Onions floating around, dead fish or animals, diesel on the water surface etc).
Most of us use NSW without problems, and we think we have good looking tanks. You shouldn't use NSW if you can't get to a decent source of it, but I believe (I know others may disagree) that is better for the tank than Artificial salt mix YMMV.
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