KNO3 from Aquarium Fertilizer (I have not looked at GLA product line, but I would assume it is the same) is agriculture grade, provides a fertilizer value of:
13-0-44
This is the official way fertilizer is rated in the USA. The first number is nitrogen, the second if phosphorus, the third is potassium. (actually it is a little bit different, but the concept is right) These are % by weight.

So, KNO3 is...
13% N
0% P
44% K

13+44 = 47.
that leaves 53% by weight oxygen.

There are no additives, no fillers, no binders.

 

Yes most aquarium ferts are going to be ag grade and will be in the 99% range. The stuff I sell is 99.5%(if I remember correctly). Im not positive but I dont think lab grade stuff is even 100% though it will be closer to it, but comes at a much greater cost. Im not sure what you are looking to use the kno3 for but if its going in an aquarium the ag grade stuff will be perfectly fine.

 

It's very unfortunate I got great answers here before the company even understood my question.... Tanks guys.

 

Nothing is truly 100%, even lab grade reagents. You can get some chemicals as pure as 5 nines (99.999%), at great cost.