A water softener simply changes certain types of molecules/ions to another type. It will not reduce the number of dissolved solids and the water will still cause problems in a reef tank as there is all kinds of junk in our tap water supply.

You can go to Home Depot/Lowe's and buy a simple Reverse Osmosis filter system that will remove most of the stuff and should be sufficient. Don't buy one of those cheapy water filters as they usually just remove the chlorine from the water. If you want a good system and have the money for it, check out Airwaterandice.com or Aquaticreefsystems.com for systems and more info. These two companies have the best products and service and are familiar with the saltwater/reef keeping hobby. The systems may seem expensive at first, but they are one of the best investments you can make if you intend to stay in the hobby for awhile.

Also, if you do buy a system, also get a TDS meter. They usually don't cost that much and they will help tell you when it's time to change out the filter cartridges.

Hope that helps,
Chris