Dosing nitrogen?

[bman]

Through the constant trial and error process, I came up with a formula for my 20 gallon tank to get my nitrogen level at 20 ppm; it was 1 ml (Seachem Nitrogen) daily, 3 ml after weekly 40% pwc. This worked very well for the first month I started with this regime. After the that month though, my nitrogen level started creeping upward and after two additional weeks, it topped at 60-80. I stopped dosing altogether and did a pwc to get it to around 40.

I have not added ANY nitrogen to the tank for about four weeks now and my nitrogen level has remained at 20. How is this possible that I do not have to add nitrogen anymore?

I still dose potassium(The same regime as the nitrogen), but have decided to scale that back to just 3 ml weekly. Am I correct in assuming that the potassium level should be fine? I have no interest in purchasing an expensive potassium kit, and prefer to use the plants as my indicators; Which are doing well.

Side note: I do have about four plant tabs in the bottom under the big clumps of plants, and do not dose Phosphate because my tap water (I am told by the lfs) has plenty of that. I do dose iron once a week as well.

 

[Purrbox]

If plants are nutrient starved they will uptake extra of the Fertilizer for awhile when it becomes available. This is probably why you were fine at first and then later saw rising NO3 levels. With those high levels of NO3 your plants were freely able to make use of the PO4. It's possible that they may have actually used up the PO4 and that's why they haven't been using the NO3 lately. Do you have a test kit for PO4? That would be the best way to determine whether it has bottomed out on you or not.

 

[malkore]

In addition to what Purrbox has already mentioned:

If the tank was brand new when you started dosing, you had no cycle in place - thus no naturally occurring nitrate.

A month later, your cycling bacteria is established, and producing nitrate as the end product.