Right is probably the best way, but I find it easy to just mix part RO
and part tap water. I also add plant fertilizer minerals, but I don't if that is what the fish need.
Buffering the water to an exact value might be kind of hard. But you might be able to do this by varying the amount of RO
-Right or tap water that you add.
One trick I like about the RO
filter is varying the filtration. My particular RO
system ($70 ebay) has 6 stages. 3 prefilters, then the RO
membrane, then 2 more filters. There are two outputs to the filter. One that goes through all 6 stages and one that only goes through the 1st three. For the "tap water" I add, I send it through only the 1st 3 stages. This filters out chemicals like chlorine but not minerals.