I expressed this same concern as I was irritated that they don't disclose the NPK like most most companies to, however I then found this analysis of the dry content of macro algae that shows more potassium content than nitrogen. Granted, this is not Chaeto. If anyone has the profile for chaeto, please share.
Gracilaria Parvispora:
Nitrogen. 2.6 %
Phosphorous 0.08%
Potassium. 13.5%
Calcium. 0.55%
Magnesium. 1.16%
Sulfur. 4.80%
Zinc. 139 ppm
Iron. 107 ppm
Manganese. 20 ppm
Cooper. 7 ppm
Source: Subsea
That being said, It stands to reason that potassium nitrate as a nitrogen source should be a better option than sodium nitrate for certain macro algaes unless there is another significant source of potassium we are not accounting for. Also, If you do have high potassium levels for some reason, perhaps Gracilaria is a better option than Chaeto.
I was using a general purpose fertilizer with an NPK of 20-4-16 for a month or so in a mixed macro tank. I dosed to 40ppm nitrate once per weak. At the end of each week the nitrates and phosphates were both unreadable (near zero). The caulepa had growth spurts, but turned white when nitrates dropped too low. All the red algaes seem to tolerate swings better. I never measured potassium levels. I will not do this again as I found going to zero makes caulerpa go sexual. I am now dosing daily with NeoNitro and NeoPhos and Chaeto grow. When I run out of NeoNitro, I will switch to potassium nitrate. I will also start testing for potassium if it can be done economically.
Interestingly, I didn't realize I was dosing toxic levels of copper as well (0.05% Chelated from fertilizer), but the macro must have been absorbing it as none of the inverts suffered. I would not recommend this though now that I know better.
Also, that whole microbactor thing looks like snake oil to me. An established tank should already have these self sustaining biological processes.