When the tail look like it weighing him down its swim bladder.
I would issolate the fish to a container at the top of the tank if its struggling to swim.
All you can do is try a bacterial med but you need aeration in the container if you add the med.
Taken from the above link.
The third type of infection produced by Aeromonas is by no means the least. This form is often the most virulent (potent) and can be rapidly fatal. This insidious form does not produce any noticeable external ulcers. The most notable feature is a behavioral change in which the infected fish generally become listless and lethargic. Some fish may lose color or appear darker. The fish do not feed, frequently seem off balance and may sink to the bottom laying on their sides. The course of the disease is rapid, and by the time the behavioral changes are noticed, the entire population of a tank may succumb, sometimes in a day or so. This is an internal infection, with the bacteria being found in the kidneys and blood of these fish. The bacteria produce potent toxins that account for the severity of the disease.
