The behaviour is normal: ink residues always remain in the tank and on the piston seal; when turning up, the seal pulls these residues with it, which makes the viewing window appear stained. As long as the same ink colour continues to be used, nothing needs to be done. With an ink change, rinsing should be carried out with water until the viewing window is clear; the same applies before longer writing breaks to avoid drying out. A professional cleaning is only necessary with stubborn deposits or problems with the ink flow; with frequent writing with water-soluble (royal blue) ink, the problem occurs less often.