Cuz i didn't see my plecos most of the time, i tried some stuff with the light.
I put 6 LED-spots right above the stream of bubbles out of my air stones.
This creates white dispersive light,and stop the beamy effect of the LED's but was not enough to actually get them out of hiding.I was at my neighbours place and saw a good idea (which i took and made my own).He used refrigerator-bulbs(15W) on top of the tank.I used 2 and placed the on 2 pieces of wood, so you don't get direct light into the water. Put a dimmer on them, and out they came. Yesterday I watched the 4 of them at once, which was NICE!!! First time. My Xingu new polkadot came to eat in the open , it took me 6 months of waiting and now he finally comes out.