Hi Friend,
don't waste your money investing in these unit. Tap water in Singapore do not need such gadget.
If I were you, rather than spending 500-800 to buy a reasonable RO/DI unit, I would pay 100 bucks to get someone to top up the tank with natural sea water.
first of all, it can cycle your tank faster, and they are much cheaper as compare to synthetic salts !
I have been using natural sea water and many tank parameters are stable.
nitrate , phosphate are undetectable. PH is good at 8.2-8.4