A scientist wants to test whether the pH, salinity, or temperature of seawater impacts coral development. He sets up an experiment with three conditions:
Aquarium 1: Normal pH, normal salinity, normal temperature
Aquarium 2: High pH, high salinity, high temperature
Aquarium 3: Low pH, low salinity, low temperature
What factor would improve experimental design the most?

Eliminating Aquarium 1 since it only uses normal conditions

Testing one variable at a time and keeping the other two variables constant

Using one aquarium and changing the variables from day to day

Setting up the variables on a gradient in each aquarium