So my six line is usually a very active fish and swims around all day. Yesterday he was no were to be seen and he didn't even show up for feeding time. Today I lifted a rock to find him sitting there, not moving much. He swam out a little bit, staying on the sandbed, and is now just sitting between two rocks motionless. I'm afraid that my six line won't last much longer. Is this normal or should I be worried? What can I do?