As already mentioned above, it doesn't account for plants. Plants filter the water and use the nitrates to grow, so the more plants you have, more filtration in your tank. However planted tank only works if the balance has been archived, meaning the waste produced by the fish is absorbed and used up by plants in order to grow. Many new aquarists struggle with achieving the balance, which means they need a filter in order to keep water perimeters whit-in acceptable levels. Hence the website would be used for new aquarists and I thinks that it's intended purpose