That doesn't look good... seems very unnaturally swollen & bloated. Are the eyes bulging at all, too? Was it fine that day, and got bloated after eating? Or did you come home to find it this way? I've heard people use soft/deshelled peas as a relief for fish constipation, but have never tested this myself. It doesn't really look constipated though... hopefully it's just a little over-fed and will be fine! Good luck with your baby!