RO is actually better for the fish that tap being that you have consistent control over parameters long term, and the local water treatment plants can and will change things up on the fly, who knows what mother nature does to well water over time.
The GH booster from the places I mentioned is the best bang for the buck in the long run, a proven and pricey alternative would be Seachem Equilibrium, honestly it's all the same stuff which is why you can't find any info on their msds's. I guess the directions on the bottle make it worth the money but if you're testing to reach your own targets then who needs directions?
There are a lot of folks here not adding bicarb for any kind of buffering capacity, while injecting CO2, and of all the old threads here about that, no one has reported a deadly "crash". But alas I am one of the ones that still lives in fear, so I add that 1/8tspn/5gl for 3dKH and a little peace of mind, needless to say no issues. Once I get CO2 pumping strong the pH drops to 6.4 at the lowest, no further unless I have a mishap with the needle valve, in which case extra bicarb wouldn't do anything to save my fish from a CO2 overdose anyway.