Any salt brands that you buy will not contain nitrates or nitrites, since they are toxic to reef tanks.
Nitrites and nitrates are coming from too many fish, too much feeding, insufficient filtration, feeding flakes or pellets instead of frozen food, fish added too quickly, using tap water instead of RODI water, or something dying in the tank (among other things, those are the most common causes).
Since you are seeing nitrites, your bacterial bioload is not large enough to handle the fish you have in your tank. In a tank that has sufficient bacteria, you should never see nitrite spikes (nitrate spikes are a lot more common, and a lot less harmful).
If you don't have a lot of fish, or you haven't added any fish in a while, but recently saw the nitrite spike, my bet would be on something dying in the tank.