My Black Moore Help please?

My black moore goldfish has been laying at the bottom of the tank since yesterday i mean he does get up and swim around time to time but he is mostly just laying there. I keep the tank light off so algae dosent spread and when i see him laying there i turn it on and he starts to move around but then goes back right to the same place..whats wrong with him? I have 1 other goldfish in wih him.