Refractometer

It measures the refractive index of the sample — how much light bends passing through it — and the printed scales convert that into salinity and specific gravity.

The supplier does not publish a resolution or accuracy figure for it, so where a stated tolerance matters use a conductivity meter such as the YSI EC300A or Pro30.

Yes, and it has it — Automatic Temperature Compensation corrects the reading for ambient temperature, which matters because both refractive index and salinity readings shift with temperature.

No — it is purely optical. Zero it on distilled water and it is ready to use.

A few drops on the prism is enough, which makes it practical for checking transport tanks and small volumes.

Not usefully — refractometers are best in seawater and brine strength ranges; for fresh, brackish or low-salinity water use a conductivity meter.

Use whichever your records are kept in — specific gravity (1.000 to 1.070) is common in aquaria and live holding, while the salinity scale suits farm and processing records.