rapid gill movement.

My cichlids seem to breathing heavy. For the most part.... 4 out of 5 of them
seem very active. (4 one 1 1/2 inch convicts, and a 2 inch green terror). They
swim around and eat, but they seem to be moving their gills very rapidly. I do
rememeber seeing this in a book somewhere as a symptom of some kind of
condition. Can anyone shed some light on the subject. I have checked my water.
I got 0 ammonia 0 nitrite and my reading of nitrate was 40 last night.. before
i did a 25% water change.... so i'm assuming that is down also. I did use some
water conditioner to detoxify the chlorine also. So i don't think thats a
problem either. Let me know what you guys think