Upside down catfish are very interesting creatures of the night. They spend a lot of time hiding if there is not very much shelter in the tank.They are not disruptive in any way. They appreciate company of their own kind but can squabble if there are only 2 of them. These fish are fairly hardy but they get stressed out and play dead when there are trace amounts of chlorine when you add water to the tank. Their max size is apparently 10 cm, but mine didn't get much bigger than about 7 cm.
They don't swim upsidedown for ever. The one I had turn out to be the fish tank murderer. He killed fish at night. The first fish he killed was a Blue Eye Pleco (The real blue eye), and I will never forget. I just can't believe the cost of those plecos now days.
The tank murderer one is probably a Black Upside Down Catfish, regular upside down catfish only get about 2-3 inches long. Mine are peaceful and chill under the driftwood almost all day. When there was only 2 they would fight with each other, now having 6 there is no problems with that. Only time they aren't upside down is when they are getting food that's on the bottom. To help breed I do daily 30% water changes w/ colder water than what is in the tank by at least 10 degrees F and feed bloodworms
I have had two, and both swam upside down. I had the first one for over three years. I have had the second for about 6 months. He's fully grown and only swims upside down. I think that if your upside down catfish is not upside down then it's not a true upside down catfish. I have a lace catfish that used to swim upside down but doesn't any more.