what was the temp of the tank?
what was the humidity level?
are you sure that when you cleaned the tank all of the cleaning product was washed out as well?
was there any signs of stress? if they had been kept in the same container for 7 days maybe it was too much exposure for them.
otherwise there are still a few reasons why but maybe the frogs were just not as healthy as they had appeared to be.
also if the shop is dirty and unclean because he spends so much time looking after his animals what about cross contamination, dust. i do understand what you mean, but a less clean shop dosent always mean healthy animals.