Yeah man, you gotta get that ppm meter. Trust me and hear me now.....spend a little paper and get a good combo meter eventually, if you didn't already.
R/O is Reverse Osmosis water. Like the water machines you see at Walmart or the grocery store. Put the 5gal bottle in and fill'er up. Or by one for the kitchen sink at Lowes or HomeDepot, even better and cheaper in the long run. R/O water has very low amounts of elements/nutes/salts compared to water out your tap. My tap is 290 - 300 ppm, but my R/O water is 20 ppm. That's 6.7% of 300 ppm ! I can then work my nute mix and KNOW what's in 93.3% of my plants food.
But you also have plenty of ppl that use their tap and do outstanding grows!
BoomerBloomer57 is one such person.
I'm sure the Professor will drop in.