Protruding Gills on Percula Clown

[firegarden]

I have a pair of Percula Clownfish which I've had for about 13 months...I noticed the larger of the two (I assume to be female) is behaving sluggishly and her gills seem to be protruding from her sides. There are no visible parasites, and the other fish is normal. I tested everything in the tank, all test were immaculate. Any ideas to what is causing this?

 

[firegarden]

Brine Shrimp, flakes and frozen...I've never been able to get them to eat anything else. I will add some vitamins to the water!