Water changes are a good idea to get the water quality up, other than that, do whatever you can to make sure that it is not getting stressed. If it is hiding from the light there is a reason for it. Could be to much or to close for him? As long as he is not bleaching I would not worry to much about him not getting the light, he will move into if he needs it. How long ago did he split? Also, check your basic stuff, ammonia, nitrites, nitrates, Ph. Make sure nothing else is going on. If you can get a picture that could help as well.