There is no doubt that you could be over feeding your fish and any left over food could be sitting at the bottom of the pond, decaying and causing an odour. It is also possible that if you have a build up of algae at the bottom of the pond, this will also create a very bad odour. Making a one third water change from the bottom of your pond and cutting back on the feeding of your fish and feeding them a better food will certainly help. Adding or increasing the size of your filter will also be a big help.

Yours Koily, Lorraine