Really you just ought to descale your kettle regularly. Tea made from a scaly kettle will form a scum, even if you use a mesh limescale filter. Apparently York

Really you just ought to descale your kettle regularly. Tea made from a scaly kettle will form a scum, even if you use a mesh limescale filter. Apparently Yorkshire Tea for hard water might help with this, but I drink decaf so that's not an option for me.