If you can, get a TDS meter (total disolved solids). TDS are all the left over particles and impurities in the water after its filtered. The closer to 0 the TDS is, the better.
I myself use RO water from a grocery store and it wonderful. My levels are all in check and I dont have accesive algea growth like you would find with poor water quality. I would make sure it is routinly checked and maintaned as stated by others.