I have a hypothesis, and wonder if I might get input as to whether it is on track.
In a relatively large tank (220G) that is low tech but with excel, for the first several months my nitrates were low (5ppm +/-) even after a month or so without water change. Of late, they rise more quickly; I am changing water today and they are probably somewhere between 40 and 80ppm.
The tank is moderately planted (reference shot below), and I think fairly lightly stocked. My goal was to have the plants keep up with the fish, mostly, and do infrequent water changes.
Over the last several months been slowing adding more phosphate to combat some minor GSA. Which seems to be working.
Of late I am starting to see a bit slower growth, and the anubias in particular looked like they are suffering a bit, my guess is potassium (small holes in leaves, browning around holes and tips). Second shot below.
So I started looking into potassium. I have no test kit for it. I use Equilibrium which is very high in Potassium. At the rate I mix (for dGH=6) I should be seeing about 81ppm of Potassium. As I understand it, having about 15ppm is adequate, though high is not an issue.
So here is my hypothesis -- as the tank is maturing, and with infrequent water changes, could I be reaching a point where potassium consumption is outpacing what I am adding with water changes (which are only about 80-90G at a time every 2-6 weeks depending on nitrates), and in turn causing diminished plant growth rates, which in turn means less nitrate uptakes?
Of course there could be a more pedestrian answer, that I'm over-feeding and too much fish poop. But I think I am feeding less now then when nitrates were low.