The experimental groups in the experiment are the substances tested tea, coffee, and ethanol.
In this experiment a liquid that could be used for a control group is water because every substance is added into water.
To improve this experiment the students could have added missing information, such as how many drops of each substance were added to the water, and the starting heart rate of the Daphnia. They also could have checked the heart rate a minute later to see if the substance has a lasting effect on the heart rate.
An experiment that students could perform to verify the prediction that coffee