Tetra problem

Question
I have adesktop 5 gal aquarium which is filtered.  I have a pair of cories, 2 platys and 3 tetras.  The water is changed 50% every 2-3 weeks.  The problem I have is one of the tetras has been for the past month or so, just been pointing straight up and kind of bobbing.  He feeds well and swims normally when approached by tank mates.  Any clues to this odd behavior?

Thanks
Tom

Answer
Hi,
5 Gallons is a very small tank, so you should consider upgrading to a 10 Gallon soon, so you can get slightly bigger shoals, and in turn, the fish will be happier.
There is probably a lack of oxygen in the water. Raise the filter so it is breaking the surface of the water to allow more oxygen into the tank.
You should rehome the Platies, as they will need a bigger tank, and buy a few more Neons. He should act more normally when in a bigger shoal.
You should change 30 percent of the tank water weekly. That should also help.

Best of luck,
Kathryn