Depends what you're doing with the lr. If you're using it to cycle the aquarium, then uncured is fine as it will raise the ammonia level, which should happen during cycling. If you're adding it to a tank that has already cycled and has fish in it, then I would get cured lr to prevent die off which in turn would raise the ammonia level which in turn would cause the fish to have a really bad day. Uncured lr is usually cheaper and smells awful, whereas cured lr doesn't smell bad.