There seems to be a little confusion about potassium nitrate & nitrite.
Salt peter is potassium nitrite (which used to be used for curing meat!)
and probably not a desirable compound to add if you have fish in your
tank since nitrite, depending on the level, can be quite toxic.
If you do have fish in your tank then you probably don't need to add
any nitrate either - best to test this first & if your levels are under
5 pmm then you could add nitrate if you want (or just feed your fish