That's a pretty nice looking fish, especially since you're just starting! Not sure what you did to mirror it, but the best way (IMHO) is this: - in edit mode, delete everything to one side of the midline (the axis of symmetry) but not the vertices along the middle - put the cursor anywhere along where you want the middle of the fish to be and put that little menu thing so new transformations are relative to the cursor - in object mode, scale the new object along an axis (probably x) to -1