Evaluating Experimental Design in Photosynthesis Study

A study was conducted to evaluate the impact of temperature on the rate of photosynthesis in a specific type of aquatic plant. The researchers set up four aquariums with identical plants and maintained each aquarium at a different temperature: 15°C, 20°C, 25°C, and 30°C. Over a period of two weeks, the researchers monitored the rate of photosynthesis by measuring the amount of oxygen produced in each aquarium daily.

After completing the experiment, the researchers found that the aquarium maintained at 25°C exhibited the highest rate of photosynthesis, while the ones at 15°C and 30°C produced significantly less oxygen.

Which of the following statements best describes a critical aspect of the experimental design?