That product looks like it will suit your needs. You'll use pretty much the whole tub - based on your tank size and I'm assuming you have a sump.
Good on ya Jon.
Hopefully the link survives the Mods ...

carbon dosing and biomedia that host bacteria all require phosphates to be present to be effective... if your phosphate is actually 0 (I don’t know how reliable/accurate the test kit you are using is) you will need to Add phosphate to lower nitrates.

Looks like if he uses the orca labs stuff he won't need to add PO4.
The stuff is designed to work in low PO4 tanks.

Low phosphate tanks... not 0 phosphate tanks... regardless of the product bacteria (along with every other biological being) require phosphate... his tank (and mine) is likely so low in phosphate the natural bacteria present is phosphate limited. My nitrate is 15-25 (salifert) and phosphorus is 0 (ULR Hanna) I have started adding (yesterday) flourish phosphorus at 1ml/day in approx 200 gal. That would provide 0.005 PPM of phosphate. We’ll see if that makes a difference.
Cheers
Dan.