[tbittner]

Distilled water from the grocery store would be fine to use. Try to get it to the same temp as the water in the tank before dumping it in though.

You should also get a 5 gal bucket, add about 4 gallons to it and add salt to get the salinity right. Keep a powerhead in the water to make sure everything is disolved good. Salt crystals are hard on the gills. The saltwater should mix for 48-72 hours before use. You'll use this water to do water changes and the time between water changes is pretty much dictated by your water quality.

If you're replacing evaporated water, add fresh water.
If you're changing water by siphoning it out in order to get the nitrates down, replace that water with the saltwater you made up in the bucket.