I know what it is.
Its swim bladder. It is caused by poisoning/water quality How often do you change your water? How big is your tank?how many fish do you have in side. If the fish is eating well it should be fine, i also have a fish with swimbladder i suggest you go to your local pet store and tell them the problem. They would probarly give you no:13 which is swimbladder treatment. Finally do you have a filter pump? .
I hope ive come to some assistance to you.
Mary.