Theres a good chance theres a problem at the petstore.
Even lack of UVB wont kill them in a week but temps to hot or cold could but thats rare unless your literally freezing or roasting them.
Are you cleaning the tank with chemicals or bleach?
Are they on pine bark substrate? Pine sap is lethal to igs.
What are you feeding it and are you chopping it up small so they cant get food stuck?