Planted Tank Enthusiast
Join Date: Sep 2009
Location: Seattle, Washington
Most people do KNO3 because it's an extra source for K.
Generally people get:
KNO3 (for N and K)
KH2PO4 (for P and K)
and something for micros (your CSM+B)
Often there is enough K from those first two that you don't have to dose anything separately. (If you find you need another source of K you can order K2SO4.) Using CaNO3 would raise your water hardness a little, which may or may not be a good thing, depending on what you want.
I wouldn't get the N-P-K package all together; I would order them separately, getting one each of the things I listed above. That way if the nutrient ratios need tweaking you can add a little more of one or the other until you find what your tank needs. If you order them all together you don't have that option.
Edit: I see that it looks like they include a bag of each, rather than mixed together, so you would be able to adjust dosing. But they include the K2SO4 which you may or may not need, so I guess it's just up to you whether you want to wait and see if you can save a couple bucks, or just have it on hand in case you need it. Also, this package already includes KNO3, so you don't need to look for an additional N source.