David said:
If the fish seems like it is having trouble stabilizing itself and keeping its balance in the water, it's usually an indication of swim bladder complications. I really wouldn't treat the fish unless you see some definite signs of a particular disease, as you may be doing more harm than good with the meds.
Thank you David
That's the best advice I've seen on this entire thread.
NEVER treat an illness that you cannot identify because you may get rid of the very symptoms that will identify the disease, while not curing it. If this happens, the fish will get sicker and might be too far gone before you determine what actually is wrong.
While corys can get ich, they are very resistant to it and seldom do. In an ich infested community tank they might be the last fish to be effected by it or they might not get it at all. If there is white spots or patches on a cory, it is most likely a bacterial infection.
Another thing that can go wrong with treating before you have a diagnosis is that to cure ich, the temperature should be raised, while to treat bacterial infection, it should be lowered. The wrong treatment could be disastrous. And too, sometime a medicine cannot be used while another medicine is in the water and this means an additional delay while the tank is cleaned.
In this case, it does sound like a swim bladder problem, but that can also be caused by an injury. Unless you have some other reason to think it was caused by something else, a good assumption might be that it was injured while being netted and moved from the store to your tank.
Give it some time because there might not be anything you can do. Often it will heal by itself, or the fish will die. If the other fish are showing symptoms, however, something will need to be done, but their symptoms might point the way to a good diagnosis of the actual problem.
Do keep the water very clean and fresh by doing extra water changes. Keep the temperature around 75 degrees and add an airstone if possible. Make sure your chemical readings show 0 ammonia and 0 nitrites. These things will reduce the possibility of bacteria spreading if it is a bacterial infection, but they will not disturb your beneficial bacteria.
Good luck and let me know how your fish is progressing.