CORRECTION: SO SORRY I HAVE 14 FISH I WAS JUST STATING THERE SPECIES. SORRY?

he is probably stressed because the tank is overpopulated and getting ill due to amonia build up in the water. you will have to do a lot of water changes for that many fish in such a small tank. i recomend upgrading to a larger tank, like a thirty gallon. if you got a fifty you could increase the size of each of your schools to a more reasonable amount and your fish would be much more active and happy. good luck