A study is being done to test the effects of habitat space on the size of fish populations. Four aquariums of of the same size are set up with six goldfish in each one. Over a period of six months, the fish are fed the same type and amount of food. The aquariums are equally maintained and cleaned throughout the experiment. The temperature of the water is kept different in each tank. At the end of the experiment the number of surviving fish are surveyed. What is the independent variable