Thanks for the advice guys!!, all of you helped and I'll try fast him for 3 days now.. so no feeding him for 3 days? oh god it's going to be a pain to be unable to feed him T_T
but oh wells, I'll listen to your advices.. and yeah I have witnessed Riki pooping when I flashed the mirror in front of him once! I'll take that advice to mind and do it..
BTW one last question.
Is it okay if the window is 2m away from his tank (direct sunlight) not too bright? His got a dark spot in his tank to hide if needed.