http://www.aquatichouse.com/Test%20K...hydrometer.asp

The big one.

I also found this from Anthony Calfo:

I am looking for a recommendation for a Hydrometer / Refractometer for measuring specific gravity.
<your best bet/value IMO... a glass hydrometer (do spend the extra and get one of the $20-30 units). These are arguably more reliable then even the handheld refractometers>
I have the usual cheap one, but am told I should get a better one for hypo salinity treatments in QT tanks, etc. If
you have a link of one or can comment on the refractometers on marine depot (around $70), I would greatly appreciate it.
Thanks, Andy
<the cheap hobby grade handheld refractometers are good... but not as great as one sometimes here's about. Reliability in refractometers is to be found in table mount models (not influenced easily by temp) and on lab grade equipment. Refractometers that are made cheaply overseas (imported $15-25 and retailing $50-80) are not always as good as you might hope for them to be. Whichever you choose... simply avoid using the plastic hydrometers as a primary. Anthony>