I don’t really know about that one. Have you tried blasting it with oxygen? I do this fairly regularly when I can’t figure out what else to do. Clean the tank as much as possible. Empty half the water. Let the filter run so that the water is returned above the surface level. Just leave it like that until you can’t stand to wait any longer. Turn the filter off and take the filter apart and clean it in the tank water you removed in the first place. Essentially it is just a normal water change with a huge churn of water in the middle.