Yes, get actual salt meant for saltwater tanks. While Aquarium Salt may be dried sea salt, it will not work its way back to the same composition it came from. Instant ocean is a brand that would work, there are others as well.
If these F8's are already in brackish at the fish store then make sure you match SG's (you will need something like a hydrometer to measure you SG when mixing change water). If these F8's are currently in freshwater take the change to brackihs very slowly, no more than 0.002 SG per every few days at most will be safe.