I feel like I'm taking crazy pills when I read this thread.
Can we all at least agree that one plant sucks up less ferts than two plants over the course of a week? This also means that 1ppm of nitrate will last longer in a tank with one plant vs one with two plants. In fact, it might even last twice as long.
I believe op is trying to say this. He is referring to carpet plants in a square tank vs a rectangle. All this is saying is that there is more plants in the rectangle tank than the square. Or to wrap this up, the rectangle has more plants consuming more ferts than the square tank. 1ppm will last less as long in the rectangle tank because it has more plants.
More plants = ferts dissapear sooner. Even if they stay at the same ppm after a fert dosing.