Tap water contains all sorts of things that are nasty for your tank. It contains nitrates which will cause ugly algae to grow and will prevent you from keeping any inverts, like corals or shrimp.
Tap water also contains copper with is lethal to inverts in tiny tiny tiny amounts.
Get a good RO/DI filter (around $150) and it will be the best money you spend in this hobby. Or you can buy distilled water from the store, which is fine but hauling water is very annoying long term.