You mention "other catfish," do you have a small group of this species, or is this fish the only one?
Do you have chunks of wood, caves, or similar so he can get inside and be "upside down?" I can't tell from the two photos what the decor in this tank is like.
Age is certainly a consideration, or stress (answers to above will help sort this out). There are a couple other points to keep in mind however, that generally apply to most all fish. First is, do not use pH down or any adjusting solutions with fish present. A stable pH is much less problematic for fish than one fluctuating up and down. The pH is connected to the GH and KH, and these buffer it to prevent fluctuations. London has hard water, so you are never going to get the pH down permanently without reducing the GH/KH, and that is another complex issue.
Second point, never use medications unless you are fairly certain of the issue (disease), and the intended medication is the most effective and safest. Substances added to the tank water will get into the fish and this causes additional stress at the very least. Diagnosing fish ailments is often extremely difficult. For example, if this were ich [I am not suggesting for one moment that it is, I don't know] the safest treatment would be to raise the water temperature to 30C/86F for two weeks. This can easily be tolerated by most of the fish we keep in tanks, and it is significantly less stressful than using any so-called medication.
