I know that they normally stay hidden when the lights are on (or during the day) and come out at night. I put 12 in my 10g tank and they always were out, day or night, light on or off. I didn't know why but I didn't complain-I love watching them. Anyway I put 4 of the 12 in my 20g tank and I can never find them unless the light is out. They're always hiding. Wonder why they didn't mind the light in the 10g tank but they don't like it in the 20? They're both flourescent lights.