Wasserpest, I do add artificial "flavoring" back into my RO water... specifically I mix in about 1/5 tap water with 4/5 RO water. It's not exact, just based on some random line on the pitcher that I use for water changes, but it maintains a very consistent hardness and pH.
With my ferts and water changes, the water chemistry does not fluctuate enough to produce any noticeable results with my testing kits.
There are many other residents in this tank, including Neons, Tetras, and other Ottos. If you don't count the crappy guppies from PetCo, these Ottos are my first fatalities in almost a year of having residents in this tank.