This is a sticked workup thread on how to get it what to do etc... YOu need to let it age for 24hrs to 48hrs to allow for dieoff. You should not use the water after a rain to avoid contaminents rushing from the surface to the ocean and you should avoid it during Red Tide.
The water is lacking in calcium and magnesium so you do need to add traces to your water before changing. Basically all the guys out here with big tanks use ocean water, you just have to haul it yourself! Plus all of the fish stores here use that water in their tanks...hard to argue with free!