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For a saltwater aquarium, keep fish-only tanks at a specific gravity of 1.020–1.025 and tanks with invertebrates or corals at about 1.023–1.026. A reef tank target near natural seawater—about 34–35 parts per thousand (ppt), or roughly 1.026 specific gravity—is common. Choose a target suited to your livestock and keep it stable; top off evaporation with fresh purified water, not saltwater.
Choose a target based on what lives in the tank
Different references give slightly different ranges, so treat them as practical guidelines rather than competing exact rules. Petco recommends 1.020–1.025 specific gravity for fish-only aquariums and 1.023–1.025 for aquariums with fish and invertebrates. Bulk Reef Supply gives broader ranges of 1.017–1.023 for fish-only systems and 1.023–1.026 for reef systems. SeaWorld describes 1.020–1.026 as a proper marine-aquarium range.
| Tank type | Practical target | Guidance and implications |
|---|---|---|
| Fish-only | 1.020–1.025 specific gravity | Petco’s range; Bulk Reef Supply gives the broader guideline of 1.017–1.023. Use the needs of your fish and your established tank as the deciding factors. |
| Fish with invertebrates | 1.023–1.025 specific gravity | Petco’s range for fish and invertebrates. |
| Reef or coral system | About 1.023–1.026 specific gravity; approximately 34–35 ppt is a natural-seawater reference | Bulk Reef Supply’s reef guideline is 1.023–1.026. The University of Florida IFAS recommends 34 ppt or 1.026 specific gravity for a reef tank; SeaWorld equates 35 ppt with about 1.026. |
As a general rule, reef and invertebrate systems are kept closer to natural seawater than many fish-only tanks. Livestock matters: follow species-specific care guidance where available, and avoid moving a tank abruptly toward the top or bottom of a broad range.
Understand the salinity measurement
Specific gravity and ppt are two different ways to describe salinity. Specific gravity compares a liquid’s density with water’s density; ppt expresses the amount of dissolved salts per thousand parts. They are not interchangeable numbers, but seawater at 35 ppt is approximately 1.026 specific gravity, according to SeaWorld. Readings can depend on the instrument and its calibration, so use one scale consistently and interpret it for seawater.
#1 Best Overall
- Measure salt water or salinity of water, ideal for aquariums and marine monitoring.
- Measures on 2 scales, Specific Gravity (D 20/20) and parts per thousand.
- Features automatic temperature compensation.
- Comes with hard case, dropper, screwdriver, user manual and cleaning cloth.
- The main part is made of chromium, it's very durable.
Measure accurately and track changes
A calibrated seawater refractometer is a practical choice for routine salinity checks. Petco identifies refractometers as the better choice for accuracy compared with hydrometers, while a marine aquarium hydrometer is a lower-cost alternative. Whatever you use, follow its instructions and ensure it is intended for marine aquarium use.
- Calibrate before measuring. Bulk Reef Supply recommends calibrating a refractometer before use. Follow the instrument maker’s calibration procedure.
- Check regularly. Bulk Reef Supply recommends checking salinity one to three times per week and around water changes.
- Record the reading. Tracking measurements helps distinguish a real trend from a one-off reading.
- Make gradual corrections. If the reading is outside your chosen target, adjust gradually and recheck rather than making a large, sudden change.
For a reef-specific benchmark, the University of Florida IFAS 4-H Marine Aquarium Project Book recommends 34 ppt or a specific gravity of 1.026.
Rank #2
- MEASURES ON 2 SCALES - Aquarium Refractometer features specific gravity (D20/20) 1.000-1.070 and parts per thousand 0~100‰ , allow to get two results in different units by one test
- WIDELY USE - Salinity Refractometer measures saltwater, salinity of water or seawater, ideal for aquariums, marine monitoring, fish-keeping, and saltwater pools
- ATC FUNCTION - Saltwater refractometer with automatic temperature compensation (ATC) function, allow you to obtain the accurate result faster and easier
- EASY TO USE - Put 2-3 drops of sample on the refracotmter’s prism, towards light, read the accurate readings through the optics eyepiece directly
- DURABLE MATERIAL: The optic salinity tester is made of aluminum and ABS rubber, anti-rust, non-staining and resistant abrasion. Portable Refractometer comes with plastic case, mini screwdriver,a dropper and a cleaning cloth
Top off evaporation with fresh water
When aquarium water evaporates, the salt stays behind. Replacing evaporated water with saltwater therefore raises salinity. Top off with fresh purified water, such as reverse-osmosis/deionized (RO/DI) water, to replace the lost water without adding more salt. An automatic top-off system can add freshwater as evaporation occurs and help limit salinity drift.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep the level stable through water changes
Measure the tank and replacement water around water changes, and prepare replacement saltwater to match the aquarium’s salinity as closely as practical. Use an appropriate marine salt mix; reef systems may need a mix formulated with the calcium and alkalinity demands of corals in mind. Stability is more useful than repeatedly chasing a single ideal reading, so use consistent measurement and make adjustments in small steps.
Quick Recap
Best Value
- 【AUTOUTLET Salinity Refractometer】This salinity tester is used to measure salinity of brine/seawater, monitor and control the concentration of saltwater. Ideal for aquariums and marine applications.
- 【Measures on 2 Scales】This saltwater hydrometer measures on dual scales: specific gravity and salinity. Ideal for aquariums, marine monitoring, agriculture, gardening and educational purposes.
- 【Measurement Range】Density of 1.0-1.070 with min scale of 0.001. Concentration of 0-100‰, with min scale of 1‰ (1‰=1PPT=1000PPM, PPT means “parts per thousand”, PPM means “parts per million”)
- 【With ATC Function】ATC compensation Range (i.e. Built-in Automatic Temperature Compensation System) from 10℃ to 30℃.
- 【Solid and Lightweight】Salinity meter is made of durable aluminum, which is non-staining, non-corrosive and rustless. Also, it's compact in size with rubberized grip handle, very easy to carry.
Rank #4
- Salinity Refractometer：This refractometer measure salt water or salinity of water, ideal for aquariums and marine monitoring.
- Dual Scale: Measures on 2 scales, Specific Gravity (D 20/20) and parts per thousand. Density of 1.0-1.070 with min scale of 0.001.Concentration of 0-100‰, with min scale of 1‰
- With ATC Function：Features automatic temperature compensation.ATC：50℉-86℉ (10℃~ 30℃),No Batteries.
- Durable and Portable:The refractometer is made of aluminum and ABS rubber, anti-rust, non-staining and resistant abrasion.
- Includes：Salinity refractometer comes with a hard case, dropper, screwdriver, user manual,cleaning cloth and Portable USB LED Light easy to calibration and reading
Rank #3
- Specifically Designed Refractometer For Saltwater Aquarium --- The algorithm is made specifically for testing seawater and saltwater. This is different from a standard refractometer which is not calibrated to check salinity of aquarium water and will give an error in your readings.
- Automatic Temperature Calibration --- The calibration process should be carried out with both the room and the instrument at 68° F. The refractive index of a liquid changes with temperature. Due to the automatic temperature calibration, any changes in ambient temperature from 50° F- 86° F will not affect the accuracy of the reading.
- Easy And Clear Reading For Saltwater Aquariums And Reef Tanks --- The refractometer comes with a focus adjustment on the eyepiece, this will give you clarity of the optics and a sharp line for easy reading.
- Comes In A Solid Case Includes Pipette And Calibration Tool --- This refractometer is not only built extremely well, but it comes with a durable hard case for easy storage of the refractometer when not in use. It also includes a calibration tool and a pipette.
- Keep Your Fish And Plant Life In Optimal Well-being --- Pure saltwater is crucial for the vitality of your marine or reef aquarium. Use this refractometer to measure the salinity of the water, the target level of salinity in a reef tank should be 35 ppt.
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