Barbs with labored breathing

I have two long-finned rosey barbs in my 14 gallon tank, which have done amazingly well until today. I noticed both of them to have quite labored breathing and lingering at the water's surface more often than usual. Ammonia and Nitrates are both at 0 ppm, and I haven't had any problems with them before now. Their bodies seem fine - no inflamed gils and no ich, but I'm not entirely sure of what else to look for. It's a rather densely planted tank with no airstone. Would the lack of an airstone suddenly become a problem after months in the same conditions? What illnesses *COULD* this be? Any ideas?

Edit: Might also be good to note that my pH was a 7.6.
A large water change has yielded no change. I also placed a buffer to lower the pH slightly. The water is a comfortable 76 degrees. Gils and coloring are bright and healthy; however, I am noticing that the fins seem a bit clamped. They aren't operating as fluidly as usual. A new sympton to help with a diagnosis perhaps? These two were some of my very first fish, and I'll do anything to save them. :-/