Any water is good enough however the problems and risks you are taking by not using ro
have to be taken into account for you to decide if its worth the risk.
Drinking water is not acceptable for reef tanks. To many peoples surprise drinking water contains many contaminants and impurities. Ro
contains 0 impurities. Meaning no change for contamination.
Your drinking water could contain heavy metals, chlorine, ammonia, nitrite, nitrate, phosphate and many other major, minor and trace element that can not only feed algae growth but can stunt coral growth or even kill.
Bottom line is i myself dont take chances with my tank, too much money invested to risk on bad water which is the foundation of any tank.
If you cant afford an ro
then buy it from someone. If you cant afford ro
now then you never going to be able to afford losses because of bad water. Biggest thing you can do to be successful with a reef tank is to avoid problems at all costs, because no problem has a quick easy fix in a reef tank.