Ok, it's not the end of the world using tap water. Problem is the nutrients in the tap water will help feed nusiance algae. You have a couple choices:
1. Cycle the tank with the water you have & use RO/DI for water changes & top off in the future.
2. Get yourself a RO/DI unit - check
Products and answers for your water treatment needs.. They sell a unit called MightyMite + DI for $99 - great for your size tank. Then make your salt water from that.