Hi, welcome to Aquarium Advice.
Your little catfish is in need of a couple pwc (partial water changes). It will possibly die if it doesn't get clean water. There are toxins which are produced from the breakdown of food and waste from fish.
Please do water changes, at least 2 - 50% pwc asap. Please use water dechlorinator (Prime brand is ideal and best for a new fish keeper) If not the fish could die.
You need to get the water tested. Save a sample of the water about at least half cup to get tested at the lfs if you do not have a liquid test kit BEFORE you do the pwc, and then also save one after the 2 water changes.
Please read the link in my signature right away it will help you learn about your tank and the keeping of fish.
A tank can't really cycle in a day, unless you are taking established media / filter pads from a tank which already has grown BB aka beneficial bacteria. It takes weeks to grow a colony of BB which can process the ammonia in a tank which is caused by degrading food and fish waste.
Be careful of over feeding too. It is easy to do. I am an over feeder, but over the years I have been working on getting better but I still do it :/.
We want you to have healthy happy aqua critters. Read fast!
Edit addition: So you can bring a paper or use your phone to notate from the fish store test for before and after tests, the Ammonia, NitrIte and Nitrate, as well as pH and anything else they can tell you. GH / KH or whatever else they have to tell you.
Don't just let them tell you it is goo or fine, get a reading.