Marine accessories by various manufacturers. The most powerful algae magnets in the world by Magnavore.
Portable refractometers by Deltec and TMC. Possibly the most important accessory, and investment a marine keeper will ever make.
Most swinging arms and floating hydrometers have been calibrated at a specific temperature. The specific gravity of saltwater decreases as water temperature increases. The two refractometers by Deltec and TMC feature ATC, automatic temperature compensation, enabling you to stay on top of salinity!