Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If the NO3 test shows as little as 5 ppm, I dose.
If it shows over 20 ppm I do a water change.
There are other reasons for doing water changes, of course, but that is just the way I keep the nitrogen level at a reasonable level so the plants are OK without going too high for the fish.
If you really want to start with zero, then do 100% water change twice in a row, and get down under the substrate like this:
Pull some substrate away from a corner and stick the siphon down to the bottom of the tank. When the water gets down to the level of the substrate tip the tank up so the water runs into that corner. (Gotta remove the fish of course)
Refill (a few gallons, no need to fill it all the way) and siphon out the water again through the substrate.
Then refill. You might test just a tiny trace of NO3, but I do not think the tests are really that accurate. You are as close to zero as is reasonable to get in this system.
Then monitor the NO3 without dosing through the week. (Does KH2PO4 and K2SO4, but no nitrogen in any form). The NO3 level at the end of the week is what the fish food is contributing minus what the plants are using.
Now you can figure out how much KNO3 you need to add so the plants are not deficient.
For the next week test the water each time you dose. See what it does, and how much the plants use over 24-48 hours (before the next dose).
Of course this is a combination of fish food and fertilizer that you are testing.