Bio pellets are peeling and clumpy

[reef93]

My bio pellets after using for 6 months without adding more pellets or changing anything, the amount of the pellets are not dissolving so I can not add the new one in. The pellets are becoming clumpy and peeling off. I checked the pump, the flow is the same nothing wrong. I have stirred the pellets everyday but they still get clumpy again the next day, and every single pellet is peeling off. Some of them are floating to the surface and go direct to the skimmer. What is that ? Do I need to change them ?