Aquarists can use the effects that added salts have on the refractive index of a water solution to determine the salinity of reef aquarium water. As the salinity of seawater rises, the amount of salt added rises, so the refractive index rises. Figure 1 plots seawater's refractive index vs. its salinity. Figure 2 shows a similar plot of seawater's refractive index vs. specific gravity. These data are also summarized in Table 1. These sets of data demonstrate how aquarists can use refractive index to measure salinity and specific gravity, assuming they have a refractometer that can read in the appropriate refractive index range.