My name is ***** ***** I'm a biologist with a special interest in fish. I'm sorry to hear your fish is having this problem.
My first suspicion for what you have described is what is called 'pop eye.' This is not one particular disease, but a symptom that can be caused by several different things. It can begin with an injury that gets infected. Sometimes it is bacterial. In that case, it is recommended to treat with myxazin. Melafix is not strong enough to deal with these conditions. Since such infections are the most common cause of these eye conditions, I recommend treating with myxazin as your next step. If you can't find it locally, here is an online source:
It can also be the result of a vitamin A deficiency, but if you're feeding a good commercial food, that is unlikely. It can also be related to fluid build up. Kidney failure can cause fluid to accumulate behind the eyes. Excessive nitrogen gas in the water can be another possibility. This is most likely in tanks where there is a lot of agitation of the water with power jets. Fish do also get actual tumors, which can put pressure on the eye from behind. A less common cause of these problems is a parasite infestation. If the myxazin doesn't help, you can also treat with an antiparasitic, such as Praziquantel, on the outside chance that parasites are to blame. But you'll need to do a couple of water changes first. Don't use the two medications at the same time. If there is an organ failure, nothing you do will help. If there is an actual tumor, an aquatic vet can remove it, if you want to go that far to attempt treatment. If you want to schedule a visit with a fish vet, this link will take you to a directory:
Or, in the UK:
If you have more questions, let me know in a REPLY. I hope you'll be able to save your fish.
My 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!