Salinity is the #1 parameter to measure correctly on a reef tank. A 0.001 SG error means a 1.4 ppt salinity error - and over weeks, that drifts your tank. Hydrometers (the swing-arm units) are inaccurate by ±0.002 SG out of the box. Refractometers are the minimum acceptable accuracy.
Plastic swing-arm hydrometers (Instant Ocean, Coralife) are accurate to ±0.002 SG when new and drift to ±0.005 SG within 12 months. That's ±7 ppt error - enough to crash a tank. Move on.
A $25-50 handheld optical refractometer (look up the lens through the eyepiece) is accurate to ±0.001 SG when calibrated. Calibrate weekly with 35 ppt calibration solution ($10-15 per bottle). Standard refractometers are calibrated for brine, not seawater - this introduces a 0.001 SG offset that's usually negligible but can be corrected with a seawater-calibrated unit (BRS sells one).
Milwaukee MA887 ($80-100), Hanna Salinity Tester HI98319 ($100-150). Reads salinity in ppt directly, no eyeball calibration needed. Calibrate monthly. Failure mode: probe wears out at 18-24 months and reads inaccurately.
Apex/Hydros salinity probes plug into the controller and log salinity continuously. Drift faster than handhelds (calibrate every 2-4 weeks) but you catch ATO/dosing failures the same day instead of the next water change.
Calibration solution is cheap ($10-15) compared to dead coral. Use 35.0 ppt standard calibration fluid (not RO water - RO calibration drifts as the room temp changes). Calibrate refractometers weekly, digitals monthly, controller probes every 2-4 weeks.
Mix new saltwater to 35 ppt (1.025 SG) using your primary refractometer. Then verify with a backup tester before adding to the display. Two-instrument cross-check catches a drifted primary instrument before it triggers a salinity swing in the display.