concerened about my fish

Question
i have these colourful sort off red,gold and black fish,ihave noticed that one of them has been swimming upside down for awhile now sometimes it turns back to the right position but it goes back to the up side down position and its always at the bottom of the tank.whats wrong with it?  

Answer
Hi Hannah,
  Your fish seems as if it has a failed swim blatter. It causes permanet damage but your fish can still be healthy and live long. If not then it could just be it's time to go to fish heaven.
with hopes that your fish will be well,
                        Samantha