If you're in an agricultural area, there will likely also be phosphates in your water, in addition to the nitrates, etc. I think RO water is going to be your best bet. Otherwise, you'll end up with algae soup and dead corals.
In a 10g tank, you really only need to do a 10% water change. If your LFS sells premade saltwater and RO water, it's probably worth buying from them. Otherwise, something like the Cullligan machine at the grocery store will work (assuming you don't want to buy an RO unit).
If you do go for an RO unit, you'll have the added advantage of making drinking water, without all the chemicals, for your family (just do RO, not DI for drinking water).