There are so many variables that I hesitate to try to guess. For future reference, aquariums need to be set up weeks in advance of when you populate them with animals. It sounds to me like you might have been short on research before you bought the aquarium and animals. I would advise you to get a good beginners book on aquarium fish - that will tell you how to go about setting up an aquarium. If you want to keep amphibians, keep in mind that we always tell people never to keep frogs with fish. That alone could have been the problem.