In my old setup I had some nice driftwood some fake plants and sand when they were young. As soon as they were bigger I took out the plants because they didn't need any place to hide. To make then feel more comfortable make the tank as dark as you can. They don't really like bright light. When I first got my p's everytime I turned on the lights they scattered and slammed headfirst into everything. So I covered one bulb with aluminum foil with holes poked randomly into the foil to simulate light breaking through overhanging tree branches. It made them a lot more outgoing and they constantly cruised the tank even when I was right in front of the tank.
Hope this helps