Mr. Cat, my upsidedown catfish.

[grub]

Is he meant to be never seen? I have only seen him once, and that was when I first brought him?

[Little Embers]

They are nocturnal, so they are far more active at night and rest during the day, often under leaves or decor etc.

If he is in the same tank as the clown loaches that are turning black, you may want to try and find him, just to make sure he is OK. How long ago did you buy him?

[gm72]

I have a pair in my 220 tank. They are seen mainly at night. A little more information would be helpful though. Let us know your tank size, inhabitants, etc.

[toddnbecka]

Also, which species of "upside-down catfish" is it? If it's the small, common S. nigriventris they're a schooling fish, and more active/visible in groups.
It also depends on how much cover is available, what other fish are in the tank, etc.

[Crispy]

I have one catfish that I see about once every 3 months... (my raphael)... it's not uncommon for them or the synos to hide all day.