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Hello, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
From your history, I am quite concerned about this wee fish. It does sound like you have used a range of treatments that would be indicated for swim bladder disease. To see a lack of resolution, does raise concerns that this may not be a constipation induced swim bladder issue or perhaps this change in buoyancy is secondary to another issue. Furthermore, the absence of feces for months is a wee bit of an anomaly. If he has been eating but not passing feces for such a protracted length of time, we'd have to question why he didn't suffer further constipation complications like GI rupture and potentially death. Therefore, it does raise concerns whether perhaps he is passing diarrhea (which can be a sign of GI parasites) that you haven't visually appreciated.
Therefore, to start, you do need to consider checking water parameters for this tank if you have not done so in the past 24 hours. Testing water parameters is like taking a photograph, it will only reflect the period of time when the sample was taken. And since a tank's dynamics are always changing (as we add food, the fish urinate/defecate, etc), we'd want an up to date idea of the tank's health. When testing water parameters, the baseline information we'd want would be the levels of ammonia, nitrites, nitrates, and pH. There are test kits that can check these for you or you can take a sample to your local aquarium store. Most are equipped to test water samples and often do so free of charge. Depending on the findings, this will rule in or rule out whether tank issues could be stressing this fish and predisposing him to these health issues you are seeing. And it goes without saying if something was amiss, we'd want to address that to start.
Now you treatments for constipation were spot-on. The only addition to the treatment for constipation I would suggest would be Daphnea from the aquarist as this can be used as a 'laxative/clearing' agent. As well, if we are trying to rule out full guts as a reason for compression, do pay mind to the water temperature, as colder water (below 55 degrees F/14 degrees C) can impede active gut movement.
Now if your aforementioned treatments and this laxative suggestion do not get things moving within his gut, then we have to consider that his buoyancy problem is not due to a distended gut (full of feces) compressing the swim bladder. And if that is the case, then we have to think about what else could take up space within the abdomen and put pressure on both the swim bladder and the GI (since both are non-rigid structures that can be squished --and when this happens we have buoyancy issues as well as an inability to get feces past the compressing culprit). In regards XXXXX XXXXX these differentials would be, we can see this with both abscesses and tumors within the abdomen. As well, if there is a growth or infection within the swim bladder this could manifest in both a dysfunction of buoyancy and GI compression leading to struggles in passing feces.
Ideally, with this fish being such a chronic case, I would consider seeking out a fish vet for this wee fish. That is because an xray of his swim bladder (to rule out infiltration with tumor or infection both in the swim bladder and around it) could really shed light on the root of the problem. Alternatively, some vets will be equipped to sample the contents of the swim bladder. This can then be cultured for infectious causes and examined under the microscope to uncover the nature of the material extracted. Depending on the findings, an appropriate course of treatment can be initiated, an overall prognosis appreciated, and it would give your wee one the best chance in this guarded situation.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!