Just to be clear that we're talking about the same thing - was this one of those swing-arm "hydrometers", or a proper glass floating hydrometer?
In my (limited) experience I have found that a "professional" glass hydrometer (such as the Aqua Medic Marine Control can be just as accurate (if not more so...) as a refractometer. Those little plastic swing-arm jobbies are not very accurate (more correctly, repeatable...) as their moving parts tend to be affected by salt build-up. They must also be held perfectly vertical ("waterpas").
On a general note, one should never test water taken directly from the tank's surface, especially if the tank does not have a weir-type overflow, as the oily "proteins" accumulated on the surface would make the hydrometer float higher than it should.
One should also not leave a glass hydrometer floating in the tank permanently - calcium build-up, algae, and other growths would soon change the bouyancy of the hydrometer.
Hennie
