Somethings wrong with your test kit. NO3 test kits have problems at 0-20ppm range with good accuracy.
I would not trust your kit.
I would trust knowing what the source water has to begin with, call the water company and ask, and dosing roughly 1/4 teaspoon per 33 gallons will give you about 5.5ppm of NO3 and about 3.5ppm or so of K.
If the kit is off by 1 ppm or so, then that's fine. But 2-4+ppm is not. Yours sounds much more than this.
It's relative unlikely for your plants to use 5.5ppm in one day. Even a healthy tank with everything and loads of light etc, will only consume this perhaps in 2 days.
Dosing 1/4 teaspoon 3x a week, maybe 4X if the last dose is right before the water change should more than supply the tank's N needs. Note: this doesn't include any fish/critter waste sources either.
You can dose higher, maybe 1/3 or 1/2 teaspoon 2-3x a week.
But 7 tsp means 150ppm of NO3, maybe a 30 day supply of NO3, no way the tank would use this before your next water change.
Speaking of which, you might want to do a 50% weekly and try the above routine of 3x a week with 1/3 to 1/2 teaspoon if your fish load is light.
Even with fast growers and high light, it's going to be very difficult to run out of NO3. It won't matter what the test kits say iof the tap doesn't have much NO3 and the KNO3 you have is pure.
If you add X amount(in grams) to the water, you will have a certain amount of mg/l. That is how chemical known reference solutions are made(pH, NO3, Ca etc).
This method bypasses the test kit and goes directly to the reference solution and guesses a little midweek etc...but this is pretty close and accurate for the plants and uptake rates. The 50% weekly water change make sure nothing builds up.
Regards,
Tom Barr