Best way to get soft water is to use RO, or distilled (driniking water like poland spring).
Add a dash of instant ocean to add the trace elements back in, and you're good to go!
You can buy an RO filter, or buy RO water from Skipton.
They have great systems at unbelievable prices, and you can get even more of a bargain if you bid on one of their eBay auctions. I got a nice 4 stage RO/DI for $130. It's an investment but if you're even thinking about keeping saltwater fish it's a great investment.