Doing the test in a single tank, with water mixing between the two areas would not be a good test. Algae could start up on one side, then move to the other in the water. Green water and GDA would migrate easily between the two halves. And, if the plants grew faster on one side than the other, you would have to be very careful to have very good water circulation back and forth between the halves or the faster growing side would deplete available nutrients for the other side. I think two separate tanks would be the only way to do this.