science fair fish project

Question
I teach 2nd grade. Recently I had a Beta fish visit my classroom and it died. We are studying animal needs and habitats in science. My students are curious why the fish died. We made some guesses and now I'd like to turn this curiosity into a science fair project. I'd like to study goldfish or guppies in similar containers to determine what they need to survive in the container. Can you help me turn the Beta fish experience into our science fair project tying it in with the science theme (animals-needs and habitats)we are currently studying?
Thanks very much!

Answer
I can help you with this.  I just need to know how much time you have to prepare for the experiment and how long you wish to run the experiment.  Also, does the air temperature in the classroom drop drastically during the night; and do you have access to a fish store that supplies DI/RO water?