You can get away with using treated tap as long as the parameters are ok for a FOWLR
( keep an eye out for unwanted algae), but in the long run RO
is best, especially when you get corals. Get plenty of live/base rock, it'll make your fish more comfortable and give you places to put coral later on.
Research!! Make sure your fish will get a long well and stock lightly, especially if you want corals later, water quality needs to be perfect. Also, make sure you add a QT tank onto your list, its invaluable. You NEED one, trust me
It will save you a lot of heartache.
They do make nano skimmers, you might want to try it, it certainly wouldnt hurt, though ive heard you dont need them on a nano, i'll let the nano experts elaberate on that.
Keep a close eye on water parameters. I know my 10 gal
QT changes VERY quickly ( and the 55 gal
is almost always stable).