Is it a sucker mouth catfish? Or normal catfish? And if it's nocturnal it will most likely swim around at night. Or if it feels safe could come out In The day. I find that if I tread carefully and make no sudden movements or noise or vibrations when creeping into the room. ( and I stay about a metre or so away so they can't see me) they don't know Ive arrived. So they carry on as normal. I feed mine in the morning, there Is a routine, when I open the lid the fish come out to the front and up the top, when I close the lid the plecos will come out and eat. But the rare plecs won't come out all the way if they can see me.