Fish swimming vertical

Question
We have a ten gallon tank that we have had for a year with two mollies and six red tetras.  They have all been doing fine but today one of the tetras has began swimming in a vertical line and appears to be struggling to swim.  The other tetras are bumping him.  Any clue as to what is wrong??  I changed a portion of the water and the filter today and added a tablet to be sure the water was balanced and to clear up any sickness.  Everyone else is doing fine.  Thanks for your help.

Answer
Hi Sheri;

Fish will naturally pick on others that aren't acting normal. It's just what fish do.

It's hard to say what could be wrong with him though. sometimes fish just develop internal problems that we can't see. It could be organ failure, internal infection, stroke, cancer, etc. You could put him in a little baby fish breeder net that hangs on the inside of the tank. Maybe if he gets some rest he would feel better. Sometimes treating with an  antibiotic such as Maracyn Two will cure an internal infection that we can't see. It's longshot though.

I wish I could help more...

At Your Service;
Chris Robbins