Salinity and specific gravity: testing, targets and drift

What salinity and specific gravity actually measure

Salinity is the direct measure — the concentration of dissolved salts in your water, usually expressed in parts per thousand (ppt). Natural seawater sits at roughly 35 ppt. Specific gravity is the practical stand-in most hobby test equipment actually measures: a ratio comparing your water's density to pure water at a reference temperature, shown as a decimal like 1.025[1]. The two move together and both describe the same underlying property, but specific gravity is what your hydrometer or refractometer displays, and it's temperature-sensitive in a way raw salinity isn't — worth knowing before comparing two readings taken at different water temperatures.

Refractometer vs hydrometer

A hydrometer, most commonly a swing-arm type in the hobby, floats or pivots to a reading based on your water's density. It's inexpensive and simple to use, but widely reported to be the less accurate and less consistent of the two common options — sensitive to trapped air bubbles, temperature, and the mechanical wear of a swing-arm mechanism over time. A refractometer measures how light bends passing through a thin sample of your water, and is generally considered more accurate and more repeatable once properly calibrated[2]. Most experienced reef keepers rely on a refractometer for anything beyond a rough check, keeping a hydrometer, if used at all, as a quick secondary reference rather than the primary reading.

Calibrating and maintaining a refractometer

A refractometer is only as accurate as its last calibration. Calibrate against a known reference solution — either a commercial calibration fluid at a stated specific gravity, or RO/DI water itself (which should read 1.000, effectively zero salinity) — on a regular schedule, not just when a reading looks suspicious. Between calibrations, clean the prism after every use and let a sample reach room temperature before reading it, since testing water straight from a warmer or colder tank than the room can skew the result. A refractometer that hasn't been recalibrated in months is a common, easy-to-miss source of a reading that looks consistent over time but is consistently wrong.

Target ranges

Reef tanks generally target the higher end, closer to natural seawater, since corals and other invertebrates are commonly reported as less tolerant of salinity below natural levels than many fish-only livestock. Within either range, consistency over time matters at least as much as which exact figure you choose — a stable 1.022 is generally a better outcome than a reading that wanders between 1.020 and 1.025 while nominally averaging somewhere "acceptable."

Why salinity drifts upward over time

Evaporation removes pure water only — the dissolved salt stays behind, concentrating in the water that remains. Left unaddressed, this alone would push specific gravity upward continuously between water changes. The single most common way keepers accidentally accelerate this drift is topping off evaporated water with pre-mixed saltwater instead of plain purified water: doing so adds more salt on top of what's already concentrating, compounding the effect rather than correcting it. A consistent top-off routine using plain RO/DI water, ideally automated with a top-off system for a tank prone to noticeable evaporation, is the standard way to keep salinity from creeping upward between water changes.

Correcting a drifted reading safely

If you catch a salinity reading meaningfully outside your target range, correct it gradually rather than all at once. Marine fish and invertebrates depend on stable external salinity to manage their internal fluid balance, and a sudden shift is more consequential for them than for most freshwater species[3]. Adjust across several days with small top-offs or partial water changes, retesting between each step, rather than a single large correction. Our RO/DI Water Mixing Calculator is built for freshwater GH/KH dilution specifically and doesn't cover marine salinity mixing — see our Saltwater Mixing Calculator for working out how much salt mix and purified water to combine for a marine top-off or water change batch.

Why fish and invertebrates need stability

Marine fish and invertebrates live in an environment saltier than their own internal fluids and expend ongoing physiological effort maintaining fluid balance against that gradient[3]. A sudden salinity change forces a rapid adjustment to that balance, which is more consequential the larger and faster the shift is — part of why acclimating new arrivals to your tank's exact salinity, not just its temperature, matters, and why the gradual-correction approach above applies to routine maintenance as much as to acclimation.

Common mistakes

• Topping off evaporated water with saltwater instead of plain purified water, the single most common cause of gradual upward drift.
• Relying on an uncalibrated refractometer, trusting a confidently displayed number that has quietly drifted from accurate.
• Comparing readings taken at different water temperatures without accounting for temperature sensitivity.
• Correcting a drifted reading in one large adjustment rather than gradually over several days.
• Trusting a swing-arm hydrometer as the sole reading for a reef tank, where the tighter target range leaves less room for the device's typical inaccuracy.

Frequently asked questions

How AquaPulse can help

Salinity drift is slow by nature, which is exactly what makes it easy to miss — a specific gravity that's crept from 1.024 to 1.027 over six weeks doesn't look alarming on any single day it's checked, only when this month's reading sits next to where it started. Catching that trend early is a comparison problem, not a testing problem: the reading itself takes seconds, but knowing whether it's actually consistent with where your tank normally runs takes a record.

None of that happens automatically — AquaPulse doesn't measure specific gravity for you, it just keeps whatever you log somewhere you'll actually see it again next month. That's the whole value here: a record to compare against, not a substitute for the refractometer itself. It's on the App Store and Google Play, and the Water Check feature page has more detail.

Sources and uncertainty

Good to know: This guide provides general informational guidance only. It does not recommend a specific brand or product, and target ranges are widely used hobby conventions rather than a fixed requirement for every species.

• [1] Merck Veterinary Manual — "Management of Aquarium Fish" Used for the definition and physiological basis of salinity/specific gravity in aquarium management — the same source already cited in our starting a saltwater aquarium guide.
• [2] Bulk Reef Supply — specialist reef-hobbyist resource content Specialist reef-retail hobbyist source. Used for widely reported refractometer-vs-hydrometer accuracy comparisons and calibration practice. Not a scientific authority; presented as widely reported hobby consensus.
• [3] Merck Veterinary Manual — "Environmental Diseases of Aquatic Animals in Aquatic Systems" Used for the osmoregulatory stress risk of sudden salinity change — the same source already cited in our starting a saltwater aquarium guide.

What we deliberately left out: a precise numeric conversion table between specific gravity, ppt and refractive index, since these vary slightly with the reference temperature and formula used and we didn't want to imply a single figure is exact across every instrument; and ATC (automatic temperature compensation) refractometer-specific guidance, since accuracy claims for ATC models vary by manufacturer and we couldn't verify a general figure across products.

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