You can use either or BUT you need to get a dechlorinizing formula. Normally sold at petsmart or some other animal store or else the chlorine will kill your fish. And if theyre tropical youre going to need a heater for the tank aswell. If it is salt water than you are going to need a salt solution to mix into your water.
Good luck!
You really have to research before you get fish. You don't even know what water they need?? They are probably freshwater, so yes put them in tap. But you must add water conditioner to remove chlorine and chloramine.