I feel the ph is more important than the gh. The gh doesn't have a whole lot to do with your ph, the kh does. That's why I raised my kh with crushed coral.
If you did want to soften it a little, then I'd use RO or distilled water, but then you need to remember you have to use it at every water change. With your parameters, I'd probably start with only 10% RO or distilled, and see where it takes you. But remember, making adjustments will affect the fish, so do it slowly. But if it were me, I'd just put out a little more CO2 if you could, and get the ph to about 6.8.