Short answer No.. My advice is before you start, do some research on keeping tropical fish before you start shelling out money. Basically, tap water contains harsh metals/chemicals that can kill the fish so you need to add an additive to take them out (this is done every time you do a water change), however, tap water also has minerals in it that the fish need. R/O water has taken all those minerals out so you would also need a mineral additive to add to the r/o water but not one to take the harsh metals etc out.
You also need to take into account the water hardness as certain fish like certain parameters, that's why a lot of fishkeepers will use r/o water as you can control the water parameters better. I live in Essex were the water straight from the tap would be way to hard to use it alone, so i mix mine with r/o water to get what i want.
You will also need to cycle the tank before you can add fish which can take a few weeks.