Vampire crabs died

[Malin]

Hi everyone,

I'm new here and I'm writing because I have just lost two of my Vampire crabs.

One was a Vampire crab, the other a Red Devil crab.

They were fine until I moved them into a bigger tank. I made sure they were well separated in order to prevent any fights so I'm 100% sure they didn't die because of a fight.

After a couple of days since moving them I woke up one day noticing that the Red Devil had lost two legs and one claw.
The other was fine.
I moved the Red Devil into a smaller tank.
The next day I noticed that the Vampire had lost a leg too.
I took him out of the tank, suspecting something in it had caused the problem.
Since then they'd both been very lethargic and today they both passed away.

I'm devastated because I feel it's all my fault. Before moving them into the bigger tank they were totally fine.
I want to know if it's possible to identify a potential reason for their death.

I used the same elements that were already present in their previous tanks, same moss, sand, rocks, drift woods.

The only changes I made were:
-new tank
-added a new piece of driftwood
-added a soft sponge as a base for the moss

Can any of these elements have possibly poisoned
the crabs?

On a side note, I also have another tank with two Red Devil crabs. They're doing great, full of energy, walking, climbing, eating. I used the same water, same sand substrate and bought the tank in the same shop.

Thank you in advance for any piece of advice