I using the cheapest you find on the shelf. I think $1.10
I do not know what is your tank volume.
And make sure you has a efficient skimmer in your system
Before you start just take your NO3 reading. Then start your first dosage.
Use that same dosage for 3 days. Check your NO3 again.
Is your NO3 reduce? If not then double your present dosage.
And continue above process till your NO3 start to fall.
Continue with that dosage but ensure your NO3 don't fall to zero,
Unless you are aiming for ulns system.
But take note that your PO4 is not zero from the start.
If PO4 zero this dosing may not work