I can recall the time when one put some lead shot with water in the decanter and kept swilling it around and around.
The simplest method is to put half an egg cupful of domestic bleach in the decanter, then fill with warm water; give it a good shake and leave for a few hours before rinsing thourougly.
Have no fear the bleach should not harm your decanter; it never has mine. .......Ron
even better use Milton - it's used to steralise baby bottles and will certainly not harm your glass nor leave it tainted with the taste/smell of bleach.