I would stop adding new frogs to the vivarium, there is obviously something going on if 5 frogs have died.
Perhaps one of the first ones had a disease and your tank is contaminated?
I personally would throw away the current enclosure/contents and start over.

What type of water are you using/how are you treating it?
Did you use soap to clean (or how did you clean after each death)?
What items do you have in the enclosure and where did they come from? (outside, pet store, etc.)
Do you have one of those puff air fresheners or any other way for chemicals to be in the air around the frogs?