Umm...
Just reading my fishlopedia on your symptoms, theres so many its impossible to know...
If its not an infection or parasitic it could be a tumour, very rare but if your foods haven't been stored correctly, especially in damp conditions it could have a liver tumour caused by aflatoxin contamination (the most common internal tumour) depending on the form of tumour eg. whether it is cancerous, there is only a choice between quality of life or euthanasia as this is untreatable without a vet....
Other causes may be;
Vibriosis (even rarer) almost (if not) always fatal and impossible to treat....
Any of a variety of internal worms....
Google these, maybe there are other symptoms you may recognise.
If i was you (i'm not) i'd get to the nearest bookshop and buy as big and as knowledgable a book on fish disease and treatment as you can, its going to be quicker and there'll be loads of illnesses you hadn't thought of....
In the meantime, i can't remember if its currently in with other fish, make sure you clean up its faeces as quickly as possible, if these are consumed by other fish and it is transmittable you may have a repeat incident.
Sorry i can't help
Again....
Good luck
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If you have a vet that you can get dimetronidazole from give that a shot. In the US metronidazole, which is basically the same, is available over the counter. If I'm not mistaken it's illegal to prescribe or give metro to fish in the UK. Dimetronidazole is legal, but must be obtained through a vet. This medication takes care of internal protizoans, which are different from worms or parasites.
This is not uncommon in cichlids, particularly angels & discus. I keep metro on hand, and have treated plenty of angels with it. I'll give you my little copy & paste for what I do with angels & metro.
Put the angel in a quar tank, I usually use a 10 gallon. Increase the temp over a day or so to 90-94F. Treat daily with 40 mg/gallon metronidazole, with 50% water changes daily. If the fish isn't eating, don't feed for the first 3 days. After 3 days, get some frozen brine shrimp. Take a portion about the size of a few match heads, and sprinkle on some metro. You have to eyeball this one, make the shrimp look kind of like a tiny powdered donut. Once it thaws, mix it in & feed. Sometimes they still don't eat for a few days, but once they start to mouth the food & spit it out, the meds seem to get in their system real good. Continue medicating the water & food for 7-10 days.
Metronidazole is sold as flagyl, het-a-mit, and a few other names I can't recall offhand, used to treat hexamita & hole in the head. They usually suggest 20mg/gallon, this often isn't enough. Metro deteriorates in 8 hours, so you could do twice daily water changes. I've done this on the weekends when I have time.
I've also added epsom salt while using metro, this helps with any constipation issues, cleans out the digestive tract. I start with 1 teaspoon per 10 gallons of water, increasing by 1 teaspoon daily for 2 more days to bring it up to 3 teaspoons per 10 gallons. You will have to adjust how much you use when doing water changes. You may want to give this a try.