I don't think anyone is going to have thousands invested in livestock/coral running a brackish tank... not to mention, since brackish environments fluctate in salinity several times a day depending on the tide, accuracy isn't nearly as important.
I will say that I tested an old hydrometer today against my refractometer and got the same result... 3 times... but, that doesn't mean I'd recommend them as I've heard so many times not to trust them.
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