Good morning. According to your aquarium information your tank has been running for two weeks. Chances are your tank isn't cycled. Do you know your readings for
ammonia,
nitrite and
nitrate?
I would suggest doing 30 to 50% water changes daily until you have 0 ammonia, 0 nitrites and 5-10 nitrates, add some Prime or Amquel + to detox the ammonia for a 24 hour period until it's time for the next
water change. Sounds like your fish may be suffering from ammonia poisoning but without knowing your readings it's just a guess.
Here is a link concerning the
nitrogen cycle just in case you haven't read it:
http://www.fishlore.com/NitrogenCycle.htm
Best of luck.