Ok--that explains it, ammonia is the culprit as I had originally suspected. In an established tank ammonia should never be at a detectable level.
Ammonia is harmful and lethal to fish as are nitrites, keep in mind nitrItes not nitrAtes.
Ammonia is caused by fish waste and uneaten decaying food amongst other things and should be consumed in an established or aged tank such as yours.
All fish are negatively affected by ammonia but some are affected quicker to it than others and the Synodontis Catfish is one of them.
Simply stated, your fish is displaying symptoms of ammonia poisoning. Please see links below regarding ammonia poisoning.
What I recommend you do, in both tanks is change 30% of the water immediately. Once done add 1 teaspoon of aquarium salt or any non-iodized salt to the water, 1 teaspoon per every 5 gallons. The salt will help the fish to recuperate as it serves as a general tonic.
Every day test the water for levels of ammonia and nitrite. If either is detected then do a 25% water change. Continue to do this every day until both ammonia and nitrite test at -0-. Do not add any chemicals, ie ammonia detoxifiers etc etc, as they will only provide a temporary fix. Once ammonia and nitrite =-0- you should then begin to see a rise in nitrates, that's not a problem so long as their level does not exceed 20ppm on the test reading.
It would also be helpful to only feed the fish once a day as uneaten food and excess fish waste contribute to the formation of ammonia.
I also urge you to consider adding a hang on filter to your tank so as to help with the biological load. The make and model does not matter. The only thing that counts is that the filter is manufactured to filter a 55 gallon tank.
I personally prefer the Emperor 400 filter, see link below.
Please note that no medicine in the world will help a fish if ammonia and/or nitrite is present in the tank water.
Good luck and please let me know if you have any questions.