I thought it waz that type as well but its never grown it was alone in a tank when my ex bought it. It litterally has not grown all my other fish grew to 12-14 inches in the saMe length of time...no one is missing he's the smallest in the tank..he was bought from petsmart and they said they didn't know what he was at the time..at least 8 years ago so they didn't tsll us about living in a school or alone..
he has short wiskers/feelers and he's at most 4 inches. And I moved them from an 80g. Maybe he's just stunted
If he hasnt grown in that much time, its safe to assume hes stunted. Goodness knows how long petsmart had had him before.
In the future, its always best to disregard what pet store people say, especially in chains like petco/petsmart. Most of them are trying to make a sale. Always research before buying a fish, we're here to help!
As for his belly, he might have eaten some decor, catfish tend to eat first and ask questions later. See if anything is missing. It not, go with bloat and isolate him in another tank if possible and fast him for a few days. If symptoms persist, it would be good to look into other illnesses.
Also, youve probably lost your bacteria from your previous cycle, youve got lots of water changes ahead of you!
You can look into the bacteria in a bottle which will help, Prime as a water conditioner will help as well, it binds ammonia for a few days which will make it easier on you. Live plants will be a boon as well.
Keep testing your water, and read up on how to cycle your tanks. Its a great thing to know and will help prevent this in the future.
Lastly, I saw you used strips as a way to test.
In my experience, the strips are rubbish and it would be wish to invest in a API master test kit. Its liquid and far more accurate.
Hope that helps, good luck!