You should look somewhere to buy RO
water. Nearly all fish stores sell it as do large grocery stores. Most people buy a couple of 5 gallon buckets only for aquarium use. You can also get nice 6 gallon water jugs from Academy Sports. That's what I've always used. Tap water will cause all kinds of problems in a saltwater tank.
If you have some money to spend it's always best to get a little RO
filter to use at your house.