What is likely happening is that your water has a lot of positive hydorgen ions and as a result the buffer (perhaps Calcium Carbonate) is percipating the calcium as the ions combine with the carbonate giving of Co2
This will result in a milky haze (all of the calcium in the solution). Generally this will settle over time - or end up as a white crustaceous deposit on the glass, etc. The molecules are too small for the filter to remove.
As stresco mentioned - you should likely measure the paramters of the water before you add the buffer.