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?