If you have a reef system, then you should use RO
water for your waterchanges and for top off. The reason is that RO
water is about as pure a water as you can get, anything less and you are adding impurities to the tank that will fuel algae and possibly harm your inverts. Remember, when you are replacing evaporated water...the only thing that left the tank was pure water, so that is what you need to replace, FW
only for top off.