Hikari AccuProbe Aquarium Hydrometer, Mini Ideally suited for use in Nano tanks, smaller setups or sumps of larger systems Accurate to 0.001 specific gravity and salinity (PPT) measurements Durable construction for extended life Designed to give a temperature-corrected reading in warm water aquariums  68F to 85F (20C to 30C) Readings can be taken by taking the meter out of the aquarium and placing on a level surface or leveling the unit in the substrate