Marine aquarium water purification?

It does depend on where you live and the natural qualities of the water there. Your existing filter is probably going to remove the chlorine and some other chemicals but the odds are high that you need an RO unit or to buy RO water locally. All of the fish stores in my area sell it for various prices. When I first set up my aquarium I had ordered an RO unit but I was impatient and installed it with treated tap water. Within a couple of days the water was a brownish green such that you couldn't see the back of the aquarium from the front. When the RO unit arrived I drained the tank completely and started over. As far as I'm concerned RO is essential.