Please help with sick fish

[sushi*roll]

I have one lonely fruit tetra (last survivor from a bunch I got about 2 1/2 years ago).
Ever since about 4 days ago It just lays on the bottom of the tank all day long 'till I turn the light on then it tries to swim but instead of swimming forward it kind of swims up :S. The fish eats but not much as it's swimming is all messed up. There is no visible wounds or diseases. Is my fish just dying of old age most likely?

 

[achase]

Since it's a "pack" fish it might have been feeling stressed and that may have weakened it's immune system. In regards to it's odd swimming....it sounds like something is wrong with the swim bladder.

But I don't really know what could be happening or any solutions. Sorry!