Are all salt additives (to make RO water suitable for a marine tank) similar in quality or is any one 'better' than the others in that it produces the nearest to seawater?
all salt is lacking something, unbalanced somewhere, and theres always a small chance of getting a bad bucket. If i were you i would just get whatever salt thats cheap and easy for you to get your hands on, which in many places in the states is reef crystals/Instant Ocean.