I have a 20 gallon tank with an assortment of tetras, 2 swordtails and a pleco. My x-ray tetra seems like it is having difficulty swimming. It's fins are going like its trying to move but it hardly goes anywhere. It only goes a couple of inches in any direction and seems to be completely unable to swim up to the surface. It has been in the same corner of my tank for days. Water tests show all levels to be normal.

Any ideas? Thanks!