RO water out of the tap, what are best products for conditioning and restoring kh?

Another question regarding RO water and adding back in minerals.

I switched to SeaChem's Replenish, as their tech advisor said it was developed for fish that won't be in a planted aquarium and might work better for my situation. My betta's tank has only silk plants and substrate. My dKH is between 2-3 in my betta's small holding tank, pH is stable at 6.8, dGH is 2. I test water parameters every few days and all is stable. My betta looks great and is very active.

Should I add a carbonate buffer to raise KH or am I fine where it's at? Is dKH between 2-3 sufficient buffering?
I looked into using spring water to mix in with my RO water (since my tap is also RO), but the two locally available brands, Zephyr Hills and Poland Springs may not be suitable for aquarium use. Poland Springs has a KH of 2 degrees and a pH of 6.3 and Zephyr Hills has a nitrate level of 30 ppm. Zephyr Hills will give me the dKH when mixed with RO water (along with an acceptable pH of 7.6), but I don't like the idea of adding nitrates into my tank with every water change when I'm trying to reduce them.
Any suggestions?
Also, any opinions about Wonder Shells? Are they really wonderful, or a gimmick, or worse, harmful?