Even though I have added things like Roach killer to my water column, I, like others, still feel a little uneasy about it. I think just for the peace of mind, I would and do order KNO3 from a Hydroponic supply store. If you do add KNO3, you will probably need to add other things as well. Adding nitrate to the water column is kind of like cranking up the plant machine. Although there is potassium in KNO3, it may not be enough. A better source for potassium would be K2SO4.
Then your water will probably run short of phosphate, so you’ll probably need to add that, in small amounts. One good source for that is Mono Potassium Phosphate.
You wouldn’t want to add any of this without good light source and CO2 fertilizing - things can go downhill in a hurry.
All of these things can be obtained through a Hydroponic supply store, and since it will probably be mail order, you might want to get all three at the same time to save yourself some shipping charges.
-SR