You can probably get away with a single layer of mosquito netting since it is a nice, tight weave.
As far as the RO/tap mix, it all depends on what your water quality is in the first place. I have mixed in my paludariums before, but I only mixed to get my pH where I wanted it, about 6.4-6.6. That's about where my plants, both aquatic and terrestrial like it.