Adding DI to RO?

[mike31154]

RO is better than tap water, so probably better to use it than not. DI functioning properly will provide 100% purity, not 99%. Should take the TDS to 0 as I mentioned before. The advantage to this is that you're starting without any unkowns with your system. It's not possible to tell what type of contaminants or solids that 5% TDS remaining in your RO water is. I started my system with tap water and am still dealing with somewhat of an algae problem. No telling how things will turn out for you, perhaps some folks from Calgary can chime in here as to the quality of your city water.

As far as testing for things other than TDS after the RO, that would be difficult without knowing what you're looking for and most hobby test kits would not provide any useful information at the very low levels you'd be looking at. No need to add any other chemicals after the RO either, unless you're dosing your salt mix to correct any levels that it may be deficient in, such as calcium, alkalinity or magnesium. The RO and carbon pre filters on your system should take care of the nasty chlorine which many fresh water hobbyists neutralize using tap water conditioner. You won't need to do that.