I didn't hear back from you and you now appear offline. But as this is quite serious for this wee fish, I do want to leave my thoughts regarding what you are seeing and what you need to do for him.
Since he does swim properly when stimulated, this lessens concerns about balance issues induced by conditions like swim bladder disease. Instead, we have to assume that the lethargy and inappropriate positioning is secondary to a more serious issue. And if you are seeing red streaks on his belly, then this does raise our suspicions of his signs being triggered by a bacterial septicaemia
Since you noted that he is in a pond, you do need to consider moving him to a quarantine tank at this stage. Having him in quarantine will allow you to closely monitor him, ensure he is eating (especially important if you choose to use oral antibiotics in feed rather then via injection), and ensure that the quality of his tank water is pristine. It also makes treatment more practical; since doses will likely be smaller in the limited size of the quarantine tank and this will avoid stress to your pond biofilter (from antibiotic use). While relocating him and initiating treatment, you also need to check your pond's water parameters (pH, nitrites, nitrates, ammonia). The reason is because all too often we see conditions like this arise when fish are under stress (and water quality problems is the most common stress they will face).
In regards to supportive treatments, you can start by using aquarium salt. Ideally, you should use aquarium salt or non-iodised salt. With this treatment, initially use a 0.1% by adding 1 teaspoon of salt per gallon of water. You can go up as high as 0.3% over 24-48 hours (where additional 0.1% is added 12hrs apart). When adding the salt, I usually pre-dissolve the salt in tank water before adding. And then do so slowly into a high water flow area of the tank.
As well, we do have to consider treatment against the bacteria in his blood stream. Now ideally, we'd want a sample sent for bacterial culture to tell us what agent is present and what drugs it is vulnerable to. But as time is of the essence, with such severe signs, you may consider using a broad spectrum antibiotic. Therefore, it would be advisable to use a combo treatment of doxycycline/kanamycin (ideally a 2 week course) to tackle the bacteria within his bloodstream. Alternatively, since Aeromonas (gram negative bacteria) is our most common culprit you may instead choose to use erythromycin (plain Maracyn) or tetracycline, but treat simultaneously with minocycline (your Maracyn-Two) for the Aeromonas. Both kanamycin and minocycline are antibiotics, which are considered not to be harmful to the biological filter, but at high dosages, kanamycin can harm the bio filter so take the necessary precautions with your filter (and if it is out, then you need to keep a close eye on the water parameters as you treat). Also injectable antibiotics can be an option for treating this fish, but this requires a prescription from a veterinarian who has physically examined the fish
Overall, his signs are severe and in his current state he has a guarded prognosis. Still if you do want to give him a chance, then approach his treatment as I have outlined above. As well, do check your water parameters now. This is key because if there is a problem it does need to be addressed before any other fish develop sepsis.
I hope this information is helpful.
Please do let me know if you have any further questions.
If you have no further questions, feedback is greatly appreciated.
All the best,
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