If they sleep out in the open like Clownfish then after the lights have been off for a bit then shine a flashlight in their eyes and that will give you a few seconds to net them while they are stunned. If the tank isn't huge you could also pump the water out of the tank into a holding container or two, grab the fish you want, and pump the water back in. This is probably a 15-20 minute job and works great.
Mike