According to 1st page post, u are not advise to bath with water, just provide them with sand bath, they will love to roll themselves in that. My campbell love sand bath, but after that, it will be their toilet 

Wet tail can be very contagious, you must be putting them together before your first hammy died. That's why now the other also have the same thing. They can died within 24 hours without treatment for wet tail. If you wanna save your hammy, go now to the vet with your hammy before it's too late.