Sorry to read about this. I'll have to be brief in my reply, but essentially:
* the treatment/medication sounds ideal in the circumstances - finish the entire treatment course (if you haven't already) and it may need an extended/repeat course (instructions re that should be on package);
* if the sore is on the actual anus itself, my first hunch would be that he's swallowed some gravel and expelled it with the size of the piece tearing him internally and externally (my own goldfish had a tendency to swallow gravel), or otherwise that he's injured himself or been injured by the other fish chasing him - they can be quite vigorous if mating.

[ Although not of direct relevance to your particular situation at the moment, this is worth a read re long-term care: https://www.fishkeeping.co.uk/articles ... -size-life-expectancy.htm ]

Thank you so much. That actually makes complete sense I often see him nibble on gravel. I have now moved him into a tank with large pebbles instead of gravel so hopefully he won’t be doing that again! Still hard to decide whether it looks any better or not but he has only been in the medicated water since Saturday evening. Do you think he will be able to heal? He is now passing the peas fairly easily. Shall I continue to feed him these or go back to either pellets or flakes? Is there anything else I could do to ensure he recovers? Thank you so so much for your help I will keep you updated. :)

That's good that you've moved him into a tank with larger pebbles. I moved my goldfish into a tank with smaller gravel and he continued to swallow it too, then into a tank with sand which created more problems (a pouch on his side where he had ingested it and it was sitting before he expelled it - ultimately, he expelled it in a very thick format but his anus never returned to normal size again after that although it did him no harm), then ultimately into a bare-bottom tank with a few large pebbles which thankfully turned out to be the best option and the end of the substrate-swallowing episodes.

Whether or not he heals is difficult to know. The best you can do is:
* the medication you're using;
* keeping the water in tip-top condition at all times so that the wound doesn't become infected ie ensuring you have a liquid-based test kit (not a strip-based test) and keep ammonia and nitrite at 0 at all times, and nitrates no higher than 20 above whatever your tap water nitrate level is;
* keep him on the peas for a few days, then gradually re-introduce the pellets or the flakes pre-soaked in tank water (so that they don't bloat him);
* possibly consider aquarium salt (NB sodium chloride eg API Aquarium Salt) - https://www.practicalfishkeeping.co.uk ... -questions-on-using-salt/ [final Q&A in particular]. I have used it successfully with goldfish, albeit for other problems.

Hi again,
Cleaned out the whole tank and filter etc. yesterday as well as repeating the medication but it doesn’t seem to look much better. Might you be able to recommend a product I could order online? Doesn’t seem to be bothering him much but it does look quite sore and he was a little quieter today. I could try aquarium salts? Or even a bath in Epsom salts? Although I’ve never done either before so would be a bit weary and unsure where to start with it - but if it will help him I’m very happy to give it a go!
Thank you so much for your help so far.
Merry Christmas!!

Update:
It is looking worse again now. I have ordered aquarium salts and they’re arriving tomorrow. I will also do another full water change then and re-medicate with a different medication to see if that may work.
I would be so grateful if you had any other suggestions.
Thanks again :)

I can't really add anything to my previous suggestions, unfortunately, although Waterlife Myxazin might be worth a try as an alternative medication.

Good luck with the aquarium salts (sodium chloride).

Keep us posted.

Thank you I will try that next if the salts don’t work. I am adding them in little by little I’ve the next few days and I am now treating for internal bacteria because one of the images of what to look for before treating a fish with that medication looks similar to what mine has. Will keep you updated! Thanks so much.

Hi again, sorry I haven’t updated you sooner but unfortunately my fish died :( I did everything i could to help him but nothing would work. I still don’t know what it was the caused it. It could have been that he had eaten some gravel or it could have been something else. Thank you so much for all of your help anyway. Happy new year!

Very sorry indeed to read about this - but thank you so much for coming back to us to let us know (especially as you're no doubt still hurting rawly and deeply from losing him). Hopefully it is of some small comfort that you did indeed try everything you could, and that it's almost certainly unlikely that anything would have resulted in a different outcome.
All the best.