The Sera Hydrometer is excellent for regularly checking the density in saltwater.

The water conditions in the ocean are more constant than in rivers or lakes. Therefore, marine life is much more sensitive to changes in environmental conditions, so it is important that conditions are monitored regularly, and that parameters are maintained to ensure that aquarium inhabitants thrive.

A Hydrometer should be used to unsure that water being added into the aquarium is the correct density & also that the water in the aquarium is still being maintained at the correct density- Density can change due to evapoative loss for instance.