I apologize that no one has responded to your question sooner. Different experts come online at various times. I just logged on and saw your question. My name is ***** ***** I’m a biologist with a special interest in fish.I'm sorry to hear of this incident.
Usually when a goldfish's mouth is closed shut and won't open there are four possibilities:
a fungal or bacterial infection inside the fish's mouth
the fish ate something, such as a rock, that damaged the inside of the mouth
a dietary deficiency
a genetic defect.
You have already treated with an antibiotic, but it would be a good idea to try an antifungal, such as Maroxy or PimaFix. Aqaurium salt is also recommended, but you have already done that. If there's an injury, it may heal with time. If there's a genetic defect, there won't be anything you can do.
I recommend that you start feeding the fish a liquid food called Small Fry Liquid Baby Fish Food.
This is a completely liquid food designed for tiny fry, but will help Charlie to at least absorb some nutrients. Feed a 2-3 drops twice a day.
If the salt and medications don't help, there is nothing more you can do. Please don't try prying his mouth open with anything because that is very likely to cause injury or worsen one that is already there.
If you have more questions, just let me know. Unfortunately, this is a bad situation for which there aren't many solutions. I do hope Charlie will be all right.
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