Depends on your tap water, but usually tap water is a bad idea. Tap water has things in it that have either been added to kill things( chlorine, etc) or has been leached out of pipes/the ground( iron, pollution, etc) that are bad for fish. Also Tap water tends to have higher levels of nitrates( dissolved organic solids) that are harmless to humans, but will contribute to algae growth in an aquarium.
or reverse osmosis deionized water is the way to go. With a tank your size you'll save money buying a RODI
unit for use at home. A weekly water change will be between 5 and 10 gallons in a 55 and you're going to pay between10-20 dollars for that much RODI
at a LFS
. PLUS you'll need top off water to compensate for evaporation and youll want to keep at least 10-15 gallons on standby should an emergency arise. It stings a bit up front (~200 bucks) but in the long run its worth it. for the convenience and the cost.
TL;DR dont use tap water youll have algae problems, get an RODI unit