Advice for sick baby axolotl (looks bad)

Two weeks ago, I acquired 4 sickly baby axolotls to raise back to health.
They were very weak and thin.

I've been feeding them bloodworms 2-3 times a day (in the beginning they were only eating 1 bloodworm a session). Three of them have responded well. One has doubled in size and two are coming along nicely.

However, the white one, which was the basically skin and bone has taken a bad turn.
Up to two days ago, it was eating at least a couple bloodworms a day, but now it has stopped eating. I've separated it and have noticed that it hasn't pooped, but there is a black patch in near its anal region, I'm wondering if its stuck (not gravel related).

I'm not sure if you can see what I mean about the stuck poop in the photo. (For the record, that's not where the axolotls are living, it was where I was keeping them while I were cleaning their container, however, it is were I have isolated the white axolotl too).

Should I fridge an axolotl that small and weak? What should I do to help it?

Please don't fridge an axolotl unless it is absolutely necessary. You just need to persevere with this little one - try any live food that he will respond to. Daphnia are a good option and will not affect the water.

Best of luck with your baby axolotls. Keep up the good work and keep us updated.
Best wishes for a positive outcome.
:happy:

I guess I though of fridging as a fix all solution.
Unfortunately the white axolotl died (I didn't fridge).

Thank you for both you guys informative and kind posts ^^

I am sorry to hear the bad news about the white baby axolotl.
You've done so well to keep the other three going considering they were all so sickly when you got them.
Take care.