The refill ink 4-MTK25 can be used for both EcoLine Permanent Marker 21 and 25, but if the marker cannot be refilled, the reason may be that the tip has worn out during use and has therefore become shorter. The worn-out tip can no longer touch the filter in the MTK25 bottle. A further reason could be that the tip is dirty, which has a very negative effect on the capillary action. edding 25 Ecoline permanent markers are available with replacement tips.