Calcium build up is not a warranty issue and typically a difficult to turn prop will only be due to debris or calcium deposits. You would be welcome to send it in and we will do our best to free it up. My experience with calcium build up is that you will go a long way to defeating it by keeping the magnesium levels up, this disrupts the formation of precipitate. If the magnesium gets low, it is much more of an issue though a high pH, KH or Calcium will all add to it. It is also very important that the pump is disassembled before cleaning, if the drive unit is not removed, the soaking does not do much of anything to clean the tight spaces inside. You might try before sending it in, soaking it overnight in straight vinegar and then pulling the drive unit without excessive force while applying a twisting motion.