From the little info you gave it sounds as if your dissolved oxygen level is low in your water. The easiest thing to do is add an aerator to increase the uptake of oxygen in the water in your tank. That or if you have an overflow type filter, increase the overflow so there is more surface agitation so more oxygen can be taken up by the water.
this could be one of many signs....
- could be a disease .. ie. gill flukes
- could be poor water conditions... ie not enough oxygen...
- could be unstable water conditions ... things like rapid fluctuation in temp...etc..
- could be that you didnt let the tank cycle ... or ater conditions have crashed
- could be a genetic defect possibly given from birth...was it always like this?
- could be huge amounts of stress ... is there plenty of decour/ tankmates?
... and this is just some possibilities that are more likly.... need more info
hope this info helps some... and good luck