The pH value refers to the acidity or alkalinity of the water with 7 being a neutral pH value. Below 7 is acidic and above 7 (to a maximum of 14) is alkaline.
The scale however works on a log basis with 7 as the neutral point. Water between 7 and 6 is acidic by one unit but between 6 and 5 is 10 units and so on.
Therefore the difference between 7.0 (neutral) and 6.8 (your reading) is a very small change and one that is certainly within the recommended guidelines. The guideline values for pH within the EC Regulations are 6.5 to 8.5.
At 6.8 the water is only just below a neutral reading and we would not recommend that you go to the expense of any further water treatment equipment at this time.
There will be a very slight attack on your copper pipes at this level but there are no other harmful affects.