I don't want you to have to wait any longer, so I'm going to go ahead and tell you about the most likely scenarios.
While there is no way to be certain based only on an online description, there are two conditions I would suspect with these symptoms. A fish will behave this way if something is caught in its throat. A fish with something in its throat will often swim witht he ehad down from time to time, and make jerking motions to try to dislodge the object. It could be a rock, or any of a number of foreign objects. Sometimes a fish will finally manage to get the object out, and many fish keepers do nothing. However, a fish can become greatly distressed from this and eventually die of exhaustion. To help, you'd need to catch Bianca. Wrap her in a wet towel, and hold her head down. Rub the throat in one direction - toward the mouth- to see if that will dislodge the object. If that doesn't work, you can open her mouth, and using round-ended tweezers very gently
take hold of the object and dislodge it.
If you can't see anything, we have to consider other possibilities. Bianca is quite old, and odler fish are prone to many different illnesses and conditions. Tumors are one. A tumor at the back of the throat, n the mouth, or even in the head can cause a fish to be unable to close the mouth. Some such tumors can be surgically removed.
You may wnat to consider an aquatic vet for Bianca. The vet could examien her throat and mouth and determine exactly what is going on. This link will take you to a directory of UK aquatic vets:http://www.fishvetsociety.org.uk/index.php?option=com_comprofiler&task=userslist&listid=4&Itemid=84
If you have more questions, let me know by clicking on REPLY. I hope Bianca will be fine.
AnnaMy goal is to provide you with excellent service – if you feel you have gotten anything less, please reply back, I am happy to address follow-up questions. Please remember to rate my service only after you have all the information you need. Thank you!