Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I think your numbers are in the right ball park.
Have you callibrated your test? There are many posts here where people have found the API Nitrate test to be off by double or even triple! Either direction.
Test results for any amount over about 20ppm are difficult to read. Can you tell the difference between all those hot pink-rose-fuchsia shades?
You are right:
If you do a 100% water change, and the new water has 20ppm NO3 then your first test ought to be really close to 20ppm, probably not higher than 25ppm. Some water is still in the tank, even if you drained it down until the fish are flopping, and there is often some NO3 lingering in the substrate that will show up in the water when you refill.
Here is what I would do:
1) Calibrate the test.
2) Add zero NO3 from fertilizer.
3) Couple of more big water changes, back to back.
4) Test the NO3. This is the base line. The lowest you can get the NO3 without taking more measures (suggested below).
5) Monitor it daily. Any rise from here on is from fish food.
Fish food supplies reasonable amounts of N, P, and most traces. If the plants are not even using up the amount of NO3 you are adding from fish food, then they are probably also not using the P or traces from fish food.
You might just need to dose K, Fe and CO2 for a while.
Other ways to lower the NO3:
Run your tap water into a garbage can.
Hook up a filter with some nitrate removing pads, zeolite or similar materials.
Test daily until the NO3 is significantly less. Then use this water for water changes.
Run your tap water into a plants-only tank that is set up with LOTS of light, and a good supply of other nutrients. Grow emersed plants so you don't have to worry about CO2.
When the NO3 is significantly lower, use this water for water changes.
Bump: Another thought:
If the NO3 is so high in the tap water, I hope you are not drinking it, and especially no children are drinking it!