For saltwater, I would recommend that you pick up a RO
filter rather than just an RO
filter. The last stage of the RO
takes out some of the harder to remove dissolved substances that you don't want present in your saltwater mix. In other words its an additional step closer to distilled water (without any potential for dissolved copper - which was an issue with Distilled water, back in the day when distillers used to use copper tubing).
I've found that particular refractometer to be quite good.