I just bought a shubunkin yesterday, he's large in size, he's in a 90l tank, anyway everytime i put the lights on he just goes to the bottom of the tank and sits there on the rocks, is he asleep or does he just not like the lights??
Has it always had a light on it's tank (you said he was large so I'm guessing he wasn't from a pet shop...)? Or is the light a new addition? If so he may just need to adjust. If not then I would suggest trying to get some floating plants to try and diffuse the light a bit.