do you feed them floating foods? that can cause fish to swallow too much air and lead to digestive problems as well as swimbladder issues. For flakes, I always release them below the water, that way they sink in the water and dance around in the current. My fish love it, and it's much easier for them than craning their necks at the surface.
You might want to try feeding some skinned peas. The fiber in them act as a laxative and might help her buoyancy problems. Just defrost some frozen peas, take off the skin, and cut it into little bite sized pieces. My fish love them, even the carnivores. Withholding food for a couple days before and after the peas will really help clear her digestive tract, too.
I'd still like to see some numbers for nitrate values, though. It's possible to do water changes every week, but still not remove all the wastes. That's why a liquid test kit is so invaluable. It tells you if your maintenance regime is adequate.