1. Bubbles on the swing arm can give a bogus reading. I don't have a refractometer but I am aware of the potential faults of the hydrometer. Refractometers are generally more accurate, but do need to cleaned with RO and calibrated and a fairly small bump can throw them off. Aquarium Hydrometers & Refractometers How to use a hydrometer A swing arm hydrometer offers basic salinity testing. I own both (an Instant Ocean hydrometer, and an Ebay refractometer) and, honestly, I use both. Hydrometers can be skewed by micro bubbles, calcium buildup, etc. Make sure the new saltwater matches the specific gravity of your aquarium. Handheld salinity refractometers (analog) This is the cheapest and most commonly used aquarium refractometer. Hydrometers – understand the calibration temperature requirements, and take note that S.G. is not a constant number across all measuring equipment and reference sources. I know exactly how much salt to mix into my water change water, and I use a precision scale to measure the dry salt, so I don't even usually test the water before I add it into the tank. The Hanna Salinity tool ($70) has a lot of good reviews, I haven't used it personally though. Simply calibrate the refractometer, add a couple drops of aquarium water to the refractive prism and stare into the eyepiece like a pirate to get the salinity reading. To do so you will need either a hydrometer, such as the Coralife Deep Six Hydrometer, or a refractometer, such as the Red Sea Seawater Refractometer. This is a good pick if you want better measurement results. Digital vs. analog. Before changing your aquarium water, measure and record the specific gravity. And considering a good entry-level refractometer doesn’t cost a whole lot more, I personally cannot recommend a hydrometer for everyday aquarium use. If one doesn't throughly clean it after use crud can build up on the pivot point, causing a bogus reading. Simply place tank water in the hydrometer and the arm will point to the salinity level. When I finally got the water dialed in, the refractometer read 1.025 and the hydrometer read 1.020. While all refractometers do the same job, some do so through digital or analog means. Compared to a hydrometer, a refractometer is more expensive but generally smaller, more accurate, and uses a smaller amount of liquid. For professional results consider a portable refractometer or a high tech electronic salinity monitor. Go with the refractometer. To me that seems like a HUGE! So now next WC I will have to only use the refractometer and figure out exactly how much salt to mix. Most hobbyists use a simple hydrometer or a refractometer to measure salinity. Long story short, my salt ratio is all messed up now due to the hydrometer being way off. Refractometers – understand the calibration requirements and apply an offset to measure the true seawater salinity if using a saltwater/brine model. MarineAndReef.com: Aquarium Hydrometers Vs. Refractometers. If you want to make up your own mind or want a backup way to measure salinity then my top pick, the American-made Instant Ocean Hydrometer is as good as it gets. Every saltwater aquarium requires a testing instrument to measure the salinity (salt level) of the water. difference. I have a huge difference between hydrometer and refractometer readings (1.010 hyd vs 1.020 ref)… batch is now 7 days on primary… Hydrometer is calibrated at 60F and Refractometer at 68F, so temperature is not the issue for such a big difference… Reef salinity is typically between 1.024 and 1.026 specific gravity.