Yesterday I noticed my fancy goldfish (3 inch) was hovering near the bottom of the tank and is breathing heavy. I have one other little moor (1.5 inch) that is acting fine. I change the water every 3 to 4 days at 50% - which has been my routine since setting up the tank about 2 years ago. I have 2 power filters with 3 bio wheels, 2 large aerator cubes and a bubble wand. The water tests at 0 ammonia, just slight above 0 on nitrates, and 0 nitrites. pH consistently around 7.8. O2 level is around 12 ppl. They are fed a tiny pinch of sinking pellets each morning and 1 1/2 peas each night. She does have one small spot on her left rear abdomen that looks like she may have nicked it. There are no apparent signs of parasites. I'm at a loss. Any advice?