The initial fill on my 125 was tap. I didn't have a rodi
yet. Tap water contains many things you don't want in a reef tank. You can get a copy of the latest test results from the water company. It may contain phosphates, nitrates, etc. Which may be fine for human consumption but lead to algae problems in the tank.
Your tank is going to be a major investment in TIME and money. Even the best RODI
system is only a couple of hundred dollars. You'll want to use it for top off and pwc
so I would suggest getting the 150gpd membrane (airwaterice, one of our sponsors carry it in the Typhoon line of filters).