A hydrometer for a saltwater tank measures the specific gravity of your water, which tells you how much salt is dissolved. Specific gravity of 1.025 to 1.026 is the target for most reef tanks and marine fish setups. A swing-arm hydrometer like the Instant Ocean SeaTest costs about $8 to $12 and does the job for casual monitoring, while a refractometer gives you more accurate readings and costs $20 to $40.
This article explains how hydrometers work, the difference between swing-arm hydrometers and refractometers, how to calibrate and use them, and where each type makes the most sense.
What a Hydrometer Actually Measures
Salinity and specific gravity are related but not the same thing. Specific gravity compares the density of your tank water to pure freshwater at a standard temperature. At 1.025, your water is 2.5 percent denser than pure water, mostly because of dissolved salts.
True salinity is measured in parts per thousand (ppt) or practical salinity units (PSU). Natural seawater runs about 35 ppt, which corresponds to a specific gravity of 1.0264 at 77 degrees F (25 degrees C). Most marine fish and reef tanks aim for 35 ppt or specific gravity of 1.025 to 1.026.
The reason temperature matters is that warm water is less dense than cold water. A hydrometer or refractometer calibrated at one temperature gives different readings at another, which is why temperature compensation matters.
Swing-Arm Hydrometers: How They Work and Their Limitations
A swing-arm hydrometer is a plastic box with an inlet on one end. You dip it into your tank water, let it fill, and a hinged float arm rises to indicate specific gravity on a printed scale. The Instant Ocean SeaTest and the Coralife Deep Six are the two most commonly available models.
Reading a Swing-Arm Hydrometer
Fill the hydrometer by submerging it and letting water flow in. Tap it against your hand a few times to dislodge air bubbles from the arm or the scale, since air bubbles cause false high readings. Read the scale where the arm rests.
These take about 30 seconds to use and require no setup or calibration supplies. For basic monitoring of a fish-only tank, they're completely adequate.
The Accuracy Problem
Swing-arm hydrometers are notoriously inconsistent. They're calibrated at a specific temperature (usually 77 degrees F), and readings shift as water temperature changes. At 80 degrees F, a swing-arm hydrometer may read 0.001 to 0.002 lower than actual specific gravity. At 72 degrees F, it reads higher.
More importantly, salt deposits accumulate inside the plastic body over weeks of use, physically weighing down the arm and causing false low readings. This is why hobbyists who've been keeping a marine tank for years often think their salinity was always fine, only to measure with a refractometer and find the tank has been running at 1.027 or 1.028 for months.
Clean your swing-arm hydrometer with warm freshwater after every use and soak it in RO water for 30 minutes every few weeks to dissolve salt deposits.
Refractometers: More Accurate and Only Slightly More Work
A refractometer measures how much a light ray bends as it passes through your saltwater sample. More dissolved salts mean more bending (higher refractive index), which corresponds to higher salinity. You place one or two drops of water on the prism, close the cover, and look through the eyepiece at a scale with a clear line.
Why Refractometers Are More Accurate
Refractometers have no moving parts that accumulate deposits. The reading depends on optics, not on a physical arm that can be fouled by salt buildup. They also allow temperature compensation via an ATC (automatic temperature compensation) dial, which adjusts the reading to account for water temperature variation between roughly 50 and 86 degrees F.
Popular options include: - Milwaukee MA887 Salinity Refractometer (~$25 to $35): ATC, reads in both specific gravity and ppt, well-calibrated out of the box - Vee Gee STX-3 Refractometer (~$30 to $40): Higher build quality, glass prism, trusted by many experienced reef keepers - Digital refractometers ($50 to $150): Eliminate the eye-judgment step, useful if you have difficulty reading analog scales
Calibrating a Refractometer
Calibration is easy and takes two minutes. Apply two drops of RODI water (or distilled water) to the prism and look through the eyepiece. The reading should be exactly 1.000 specific gravity or 0 ppt. If it isn't, turn the small calibration screw until it reads correctly, then check again. Recalibrate monthly and any time you change the battery (for digital models) or if you drop the unit.
Never use tap water for calibration. Tap water contains enough dissolved minerals to throw off the reading by 0.001 to 0.002.
Hydrometers vs. Refractometers vs. Electronic Salinity Meters
Three ways to measure salinity, three different use cases:
Swing-arm hydrometer: Cheap, fast, acceptable for fish-only tanks where precision is less critical. Error range of +/- 0.001 to 0.002. Good for getting in the ballpark.
Refractometer: Best balance of accuracy and cost for most hobbyists. Error range of +/- 0.001 when properly calibrated. The right choice for reef tanks and any situation where you care about stable salinity.
Electronic handheld conductivity meters (like the Hanna HI98319 or Milwaukee MW600): Measure electrical conductivity and convert to salinity, typically accurate to +/- 0.5 ppt. These are the gold standard for precision, used by serious reef keepers and aquaculture facilities. They cost $80 to $200 but rarely need replacement.
Apex and controller-integrated salinity probes: If you run an Apex controller or similar, you can add a conductivity probe that monitors salinity continuously. This catches creep up or down before it becomes a problem.
For most hobbyists, a refractometer is the answer. If you already run an Aqueon or API hydrometer and have questions about the rest of your gear setup, the Best Aquarium Equipment guide and Top Aquarium Equipment roundup cover what else to have on hand for a solid marine setup.
Maintaining Stable Salinity
A refractometer tells you where salinity is, but maintaining it at 1.025 requires consistent management of evaporation. A 75-gallon tank can lose 0.5 to 1.5 gallons per day to evaporation, and every gallon of evaporated water that isn't replaced raises specific gravity.
This is why auto top-off (ATO) systems are common in saltwater tanks. An ATO uses a float switch or optical sensor to detect water level drop and automatically adds freshwater (RODI only) to compensate. The Tunze Osmolator 3155 and Innovative Marine AUQA Gadget Hydrofill are two well-regarded options.
Without an ATO, check salinity every two to three days and top off manually. In summer or in a room with low humidity, daily top-off may be necessary.
FAQ
How often should I test salinity? In a new saltwater tank, test daily for the first month. Once you know your evaporation rate and have a consistent top-off routine, testing twice weekly is sufficient. With an ATO system, testing once per week is fine to verify the system is working.
Can I use a hydrometer for a freshwater tank? Not really. Freshwater hydrometers and salinity hydrometers are entirely different tools. For freshwater, you don't need to measure salt content. If you're keeping brackish fish (archerfish, some cichlids, figure eight puffers), you'd use the same salinity refractometer used for saltwater tanks, just calibrated the same way.
What's the correct specific gravity for a reef tank? Most reef tanks maintain 1.025 to 1.026, which equals approximately 34 to 36 ppt. Some hobbyists keep it at exactly 1.025; others prefer the higher end for SPS-dominant reefs. The key is stability, not chasing a specific number.
My hydrometer reads 1.021 but my refractometer reads 1.025. Which is right? The refractometer, almost always. Swing-arm hydrometers frequently read low due to salt deposits on the arm. Clean the hydrometer thoroughly and compare again. If the refractometer is calibrated with RODI water, trust its reading.
Conclusion
For a basic fish-only saltwater tank, a clean and well-maintained swing-arm hydrometer like the Instant Ocean SeaTest gets the job done. For any reef tank, a refractometer is worth the $25 to $35 investment. Calibrate it with RODI water monthly, and you'll have a salinity reading you can actually rely on. The most common mistake I see is trusting a crusty, never-cleaned swing-arm hydrometer in a reef tank and wondering why corals are struggling.