Is my PH (6.7 -6.9) to low for the Rainbows to thrive. my PH is 7.2-7.3 from the tap then the CO2 takes it down to 6.6-6.7. they seem to stress out during water changes and some of them get a greyish slime that Melafix won't cure. If I raise the KH to then raise the PH will that help?

That's a Myco-bacteria common to Rainbows. Especially asian tank raised. In good condition they kick it very quickly. Treat in a quarantine tank with Fungus Cure and anti bacterial feed for 2 weeks . What type rainbows ? HTH Prof M

Edit: Regarding Rainbows in Low PH Mine are currently at 6.3 and breeding like rabbits, So I'm pretty sure they are relatively unconcerned.

I have several and only a few seem to get infected. The reds are the worst I started with 8 and now I have one female all died the same way that slime within six months. Next would be the Turquoise I have three of six. Also I have one Parkinson left and three McCulloch's. In my 150 gal. I have two Australian and five Emperor Rainbows that have not shown any problems since I put them in eight months ago and they are in the same water conditions.