Did it have a sucker mouth? If not, I believe that that may be one of the myriad Synodontis cats. They change in color from juvenile to adult. Check for a prominent dorsal spineI remember obtaining a very small black Synodontis albertus the size of my pinky nail, all black, and looking almost exactly like your fish. In three months he was three inches long with beautiful cream and chocolate marbling. In my ever-so-humble opinion, they're some of the most beautiful catfish available.
First, turn off the lights more. They like darkness. Second, offer wood and rock caves. PVC piping works well, too. The important thing is COVER. They really don't like coming out during the day at all. Last, if you're having trouble convincing him to eat, try delivering a clump of *healthy pink* tubiflex worms down to the bottom near a hidey-hole using a turkey baster or forceps or other long implement. You can also try mashing a clump of wet flake food into a ball and dropping it at his doorstep. Try unseasoned meat from the dinner table, as well. They can be picky, but they love a good meal, and a little novelty here and there will often excite their appetites.
Be on guard for fungal infections. Lacking scales, they are very succeptible to a wide range of diseases. Coarse, smooth gravel is preferable as well.
