I just added three pictus catfish to my tank. They all come from the same tank at the store. One is happy swimming all over. The other two are staying together on the gravel. At first I thought the bigger one was nipping at the other but that isn't the case, they lay right next to each other and when one swims a bit it goes right back to that spot with the other. I've never had catfish before and can't figure this behavior out.