Thanks everyone for trying, but i don't think he's gonna make it

He's worse today than yesterday. Yes, his foot is still there, but it's not attached to the rock. It's basically sucked itself into the shell as much as possible, from all directions. It does still react to movement, but not much, and less than yesterday. It did clamp itself closed tight when the lights went out, and opened again in the morning. The foot looks intact with no obvious tears or lesions.
I would like to at least learn from my mistake(s). Does he look unhealthy to you guys in the Sept 29th pic (far left of the 3 pics above)? Am I missing something? I know they're not supposed to gape, but in many of the ones I've seen in LFS and in pictures, the gape seemed appropriate to me. Was I somehow hurting it by feeding phyto? If it was unnecessary, was it somehow harmful? Do you think it was the Stress Zyme? Nothing else in the tank looks sick or behaves differently. To me, the downfall was so sudden and quick and I don't know what I did wrong - I'm scared to try another and if I will, it won't be for some time. The pic from the left is 48 hours prior to the two pics on the right and the difference was staggering. What do you think happened?