Let your water sit for 24 hours uncovered to get the chlorine out. Aquatic animals are not happy in chlorinated water (I know from experience!). One question to investigate could be to vary the temp of the water, one could be speed of development vs putting the aquarium in light or dark, one could be speed of development vs brand of food. Good luck!
I hate science fairs too. The kids that need a little more time learning or who have no help at home never seem to have a chance.
BTW -- 50% of her grade on one assignment? What's up with that?