rapid gill movement

Question
Hello, I have an Eclipse system 12 tank with a built in bio filter that is the only source of water movement in the tank. It has been up and running for 6 months.  I have added fish in the past and they all seemed to breathe very rapidly. I've also had to convicts that developed a bacterial infection in their gills.  After that I kept the tank running without any fish in it for 3 months.  I just added one cichlid yesterday and this fish is still showing rapid gill movement.  Their is zero ammonia in the tank.  The fish behaves normally but still has very rapid gill movement.  Is it possible that this tank is not generating enough oxygen.

Answer
Hi David,
Definitely a strange situation here. Have you tested for "nitrites" and "nitrates"? Rapid gill movement often signals pollution stress. I would be very concerned there is a water quality issue here. Traces of nitrite and high "Nitrate" are very hard on a fish's gills and could be the reason for your problem.

Is there any way you could test for these two levels? I've experienced problems like this before myself and it has always turned out to be nitrite or other pollutions issues.

Best of luck, let me know what the verdict is. :)
Karen~