10ppm nitrates isn't what I'd consider high, especially in a planted tank. I add nitrates to my high tech tank to keep the level at 15-20ppm. I seriously doubt that would be the cause. Your fish may have an illness, has he been eating regularly? Do you feed good flake or pellet foods or mostly live/frozen/freeze dried foods? I've heared about fish getting depressed when a spawn doesn't produce viable fry, maybe you need a fish shrink?