OK, I've got my head around the relationship between KH, pH, and CO2. Chuck's page was a great help!

Here is my concern. My tank runs at around 6.8 with a KH of barley 2 degrees. I would like the KH to be 4 degrees, I appreciate this will put the R/O water stored in the vat to have a pH of around 8.0. I do two water changes a week of around 80 litres (tank holds 260 litres) my tank is well stocked, these water changes help to keep my NO3 below 30mg/l. Surely adding water with a pH of 8.0 to a tank which is 6.8 is going to cause problems unless I start blowing CO2 into the R/O vat aswell prior to adding it to the tank