most catfish adapt well to having no hiding spots. If you want pimelodids they should probably have some cover(like a cave) when young, but it is not so important as an adult. In fact if you do put a cave in for an adult fish they will consider it their territory, and will defend it so. They will actually be more aggressive with the cave and may kill unwilling passersby. I would say that smaller catfish should probably have something, but larger ones as a general rule of thumb should have as little decoration in their tanks as possible. They damage themselves less, and are less aggressive towards tankmates. Other members might have inputs they wish to share on this subject. Surely most doradids would be more comfortable with a hiding spot.