Our country is reputable to being the top five countries in the world having the cleanest water direct from our tap.
This simply means that we have the least amount of impurities in our water unlike other countries where the reefers need to have RO/DI units to "purify" the water for aquarium usage or drinking consumption.
IMHO, our tap water doesn't warrant us to use costly RO equipment to further "clean" impurities in our water other than chlorine, chloramines, phosphates, heavy metals and many others.
Having a DI unit is a better alternative, I think, to remove phosphates, silicates, nitrates, etc which is recommend for a reef tank.
Fyi, the water in the following areas were tested with a TDS meter with the following result unlike other countries having high reading measurement readings:
1) Sengkang - 80 ppm
2) AMK - 75 ppm
3) Redhill - 85 ppm
4) ToaPayoh - around 80+ (can't remember)
My 2 cts.