I wouldn't do them in anything less than a 55g (Unless you have some weird shaped 40 or 50 breeder that has a 4' footprint but isn't quite as tall). They need lots of room to swim because they are so active. Even though they are smaller than the huge catfish, they still get 5"-6", and they do prefer to be kept in a group of their own kind, about 5-6 or so. I do love this catfish too though, and hope someday to have a tank for them as well.
My understanding was that they like to be in a group when young, but as they grow, prefer to be solitary and are more territorial. I only have 1 -- would love to add more if they are still prefer to school at maturity. Anyone know?
To respond to the OP, I have mine in a 33 gallon long, which is a 4' tank with the footprint of a 55 gallon but more shallow -- the Pictus never leaves the tank floor anyway so it is identical for his purposes.
Agreed - I had a group of four in a 6' tank with three 4-lined pims, and all 7 of them were all over the tank. I would say a 4' tank minimum for a group since they are very active, borderline frantic, and fast swimming fish.
To add to that, they never appear to watch where they are going -- very often my pictus will swim right into and through any other fish in the tank. It would be best to have fish with the pictus that don't mind getting run into constantly.
When I first got my pictus, I had him in a 20 gallon long and he was fine. But he grew about 3" in the first year and was soon too big for that tank so I had to move him to a larger tank.
I would agree with what others are saying, a 4" length would be best, the two that I now have are all over the place, they also see to love height going straight up the sides of the tank as well.