In the spirit of brain teasing, two questions:
1) Letís say you have a 60f and a 60p and 30ppm NO3 in each. Now you add enough plants to each tank so that the plant mass equals 20% of the volume of water in each tank and each group of plants consumes 30ppm / day of NO3 (even though the plant mass in the larger tank is more than the smaller tank). Which tank is going to run out of NO3 first?
2) Which is heavier: a ton of feathers or a ton of steel?
Maybe what we are all trying to say here is that if you have a 60f and a 60p and 30ppm NO3 in each and then add one plant that consumes 30ppm of NO3 / day, it is obvious that the smaller tank will run out of NO3 faster.