I'm not saying there is something wrong with your food at all. I'm just wondering if you feed more then one kind, e.g. pellets, and flake, or frozen. I found my fish started to thrive when I began really mixing up their foods, and creating a variety.
It sounds to me like its one of three things: bad fish (e.g. cyanide caught), water parameters (e.g. bad testkit is giving you a mis-reading), or just really bad luck.
Does your LFS test water? You may want to bring in a sample next time you stop in.
Either way, do the water change, and replace your carbon. Your tank will be all much happier for it, and you'll have some peace of mind.