Biology students designed an experiment to study the effects of nicotine on the respiration rate of goldfish. They counted the number of times the gills of a goldfish moved at room temperature (25 C). After this each lab group placed two goldfish into five 2000 ml beakers containing 1500 ml of water. They waited 10 minutes for the fish to calm down and begin to breathe normally. The students then used a dropper to place 20, 25, 30 and 35 drops of pure nicotine into the water of four of the beakers. The water in the fifth beaker had no nicotine added to it. While viewing the fish through a magnifying glass the students counted the number of gill movements for 60 seconds. . Which variables in this experiment should made constant in order to get reliable results