What to do when fish get something stuck in their mouth.

Have a soft net, fine point, not sharp point tweezers and a very soft clean rag handy.
First up catch the fish as gently as possible.
Hang the net with the fish in it in your tanks water.
Wet the rag with the tanks water lay it out flat.
Lift the net out and sit either the fish out of the net on the wet rag or sit the net with the fish still in it on the wet rag.
If out of the net, fold the rag to gently cover all of the fish bar the area you are working on, that being its mouth.
Once the area has been worked on sit the rag in the water, open the rag and let the fish swim off.