You have a leak, Nothing else would cause that problem, As it started shortly after the tank was fitted, check around that to start, wipe your hand around the joints, will not be much of a leak, as it takes months to drop, so very slight wet, look for signs of run down from the joints, Make sure all your radiators are bled of air. Check all 4 corners of radiators, even the blanking plug. But you have aleak.
SD
October 2011