Internal storage is used up by Android
Unfortunately this is a common problem for all xperia J users, including myself. I have installed many recommended apps for moving stuff from internal to external SD card but after doing this, pictures, music, videos etc I have moved cannot be played/viewed meaning you have to move them back in order for them to be viewed/ played again, however adter moving them back my files still didn't work. I have not yet come across the answer to solving this problem but I can recommend not downloading apps that move stuff to SD card as they won't work, just clear your cache as much as possible to relieve it of some memory. Hope this helps a little bit