2 best & most accurate hydrometers for aquarium (13 tested)
Instant Ocean Hydrometer : Amazon.in: Toys & Games
Capetsma Salinity Tester, Accurate Sea Hydrometer Aquarium Marine Salinity Meter for Fish Tank Pond Water Saltwater Freshwater Specific Gravity Test : Amazon.ca: Pet Supplies
Aquarium Systems SeaTest Hygrometer - Buzzard Reptile