RO&DI is different from Tropical (Freshwater), because the chemicals you are adding to your freshwater water is removing the chloride, etc... RO and DI remove silicates, phosphates etc, which are not as big a problem in Freshwater tanks, as you are not trying to replicate such a low-nutrient environment. Phosphates and Silicates cause algae(and lots of it) in a marine tank.
Cannister filters are not the best to run, if only want soft corals it will be acceptable but anything else and you will run into troubles. the problem with cannister filters is they convert ammonia to nitrites and nitrites to nitrates, but do not remove the nitrates so this leads to a favorable environment for algae.
I have blue lights on 1/2 hour before my halides come on/go off, to try to ease the stress on the fish from the quick change in brightness. I also looks cool in the evenings as alot of SPS and LPS have fascinating colors under the blue light.