Keeping salinity stable is one of the fastest ways to make a reef tank feel “easy.” Keeping it accurate starts with one small habit: calibrating your refractometer the right way (with 35 ppt solution, not guesswork).
Below is a beginner-friendly, step-by-step guide to calibrate a refractometer for reef tank salinity, measure consistently, and avoid common errors that quietly stress fish and corals.
Why accurate salinity matters in a reef tank
Stability vs “perfect numbers”
Most reef problems come from swings, not slightly-off targets. A refractometer that reads low can lead you to add salt. One that reads high can make you dilute. Either way, chasing a bad reading can create daily salinity swings that stress animals.
Safety note: If you discover your tank is far from target, correct salinity slowly. For most systems, changing more than about 0.001 SG per day (or roughly 1–2 ppt per day) can be stressful. When in doubt, slow down.
What salinity range most reefs aim for (35 ppt / 1.026 SG)
A common target for mixed reef aquariums is:
- 35 ppt salinity
- Approximately 1.026 specific gravity (SG) at typical reef room temperatures
Some successful tanks run a little lower or higher, but consistency matters most. If you keep fish-only marine systems, you may see slightly different targets, but 35 ppt / 1.026 SG is a solid reef baseline.
Refractometer basics (what you’re actually measuring)
Salinity (ppt) vs specific gravity (SG)
Refractometers often show one or more scales. The two you’ll see most:
- ppt (parts per thousand): a direct salinity unit (e.g., 35 ppt)
- SG (specific gravity): a ratio relative to pure water (e.g., 1.026)
If your refractometer has both, you can track either. Many reef keepers prefer ppt for clarity, then reference SG as needed.
ATC explained (and what it does not fix)
ATC (Automatic Temperature Compensation) helps adjust the reading for temperature changes within the device’s designed range. It’s helpful, but it does not mean:
- you can calibrate with cold fluid and measure warm tank water without impact
- you can ignore large temperature mismatches
- the device is automatically accurate without calibration
ATC reduces error; it doesn’t eliminate the need for good technique.
What you need before calibrating
35 ppt calibration solution (recommended)
For reef tanks, the most reliable method is calibrating with a 35 ppt calibration solution (often labeled 35 ppt or 1.026). This aligns the refractometer where you actually use it—around natural seawater strength.
RO/DI water—when it’s okay and when it’s not
RO/DI (or distilled) water is 0 ppt, and some instructions suggest calibrating at zero. The problem: many hobby refractometers aren’t perfectly linear from 0 to 35 ppt. Calibrating at 0 can still leave you off at reef salinity.
Use RO/DI water for:
- rinsing the prism
- quick checks if you suspect major contamination or residue
Use 35 ppt solution for:
- reef tank accuracy at your real target range
Tools checklist
- Refractometer
- 35 ppt calibration fluid
- Pipette or dropper
- Lint-free wipe or lab-style tissue
- Small screwdriver (if your model requires it)
- Microfiber cloth (for exterior)
- Optional: thermometer (for room temp checks), distilled water for rinsing
Step-by-step: Calibrate your refractometer (35 ppt method)
Step 1 — Let everything reach room temperature
Set the refractometer and the 35 ppt solution in the same room for 15–30 minutes. This reduces temperature-related drift.
Step 2 — Clean the prism and cover plate
Open the cover plate and clean the prism with a lint-free wipe.
- If there’s dried salt, use a few drops of distilled/RODI water first.
- Wipe gently. Scratching the prism can permanently affect readings.
Step 3 — Apply calibration solution correctly (no bubbles)
Add enough 35 ppt solution to fully cover the prism (usually 2–3 drops). Close the cover plate so the liquid spreads into a thin, even sheet.
Avoid:
- air bubbles
- dry spots
- too little fluid (gives patchy light/dark edges)
Step 4 — Wait 30–60 seconds for stabilization
Let the solution sit on the prism for 30–60 seconds. This helps the reading settle, especially if the prism was slightly cooler/warmer.
Step 5 — Aim at a bright light and focus
Point it toward a bright window or lamp (not directly at the sun). Look through the eyepiece and focus until the scale is sharp.
Step 6 — Adjust to 35 ppt (or 1.026 SG)
Use the calibration screw to set the boundary line exactly at:
- 35 ppt, or
- 1.026 SG (depending on your scale)
Make small adjustments, then re-check.
Step 7 — Re-check, then repeat once to confirm
Wipe the prism, reapply solution, and confirm it still lands on 35 ppt. This quick repeat catches bubbles, residue, or a rushed reading.
Step 8 — Rinse and dry (prevent salt creep and residue)
Rinse the prism with distilled/RODI water, then gently dry with a lint-free wipe.
Tip: Never store the refractometer with saltwater residue on the prism. Salt creep and film are a top cause of drift.
How to measure reef tank salinity after calibration
Sampling tips (display vs sump, avoid surface film)
Take a sample from a consistent location and method:
- Prefer mid-water, not the surface (surface film can skew readings)
- Avoid sampling right after adding saltwater or fresh top-off water
- If you use a sump, sample from a well-mixed chamber (often the return section)
Reading the scale correctly (ppt and SG)
- Use the sharp boundary line between light and dark.
- Read at the point where the line crosses your scale.
- If the line is fuzzy, you may have bubbles, too little sample, or a dirty prism.
Common reef target: 35 ppt / ~1.026 SG
Logging results to spot trends (daily swings, top-off issues)
Salinity issues often show up as trends:
- slow rises (top-off not keeping up, ATO failure)
- slow drops (overactive ATO, leaks, wet skimming)
- daily swings (manual top-off timing)
Logging makes this obvious. Reef Buddy can help you record ppt/SG and connect changes to maintenance events.
Common mistakes (and how to avoid them)
Calibrating with RO/DI water and reading saltwater
This is one of the most common refractometer errors in reef aquarium keeping. Calibrate at 35 ppt for best accuracy at reef salinity.
Not cleaning the prism (residue shifts readings)
A thin film of salt or dried sample can push readings high or low. Rinse and wipe after each use.
Temperature mismatch between fluid and device
Even with ATC, large differences can cause inconsistent results. Let the refractometer and calibration fluid sit together at room temp first.
Bubbles, dry spots, or too little sample
If the liquid layer isn’t uniform, you’ll get a wandering or fuzzy line. Use enough fluid and close the plate smoothly.
Confusing ppt, SG, and “salinity” scale markings
Some refractometers include multiple scales (and not all are ideal for seawater). When possible:
- prioritize ppt or SG
- ignore unclear “salinity” markings unless you know exactly what they represent
How often to calibrate (beginner-friendly schedule)
New refractometer: first week routine
For the first week:
- calibrate before the first use
- re-check calibration every 1–2 days while you learn its behavior
Ongoing: weekly or biweekly
A simple routine most beginners can stick to:
- weekly calibration if you test often
- biweekly if testing is less frequent and your device is stable
Always calibrate after drops/impacts or big temp changes
Any of these should trigger a recalibration:
- dropped refractometer
- stored in a hot/cold car or garage
- big seasonal room temperature shifts
- you suspect a bad reading
Troubleshooting inaccurate readings
“My readings drift” — what to check
Work through this quick list:
- prism clean and scratch-free?
- using 35 ppt calibration solution that isn’t expired/contaminated?
- bubbles or dry spots under the cover plate?
- waiting at least 30–60 seconds before reading?
- measuring in similar room conditions each time?
Cross-checking with a second method
When you’re unsure, verify with another method:
- a second refractometer (calibrated with 35 ppt)
- a quality hydrometer (less precise, but useful as a sanity check)
- a trusted local fish store test
If the numbers disagree, don’t “average” them. Identify which tool is off and fix that.
When to replace the unit
Consider replacement if:
- calibration won’t hold even after cleaning and careful technique
- the prism is scratched or the cover plate won’t seat evenly
- the scale is consistently hard to focus or read
Comparison: 35 ppt solution vs RO/DI water calibration
| Method | Best use | Pros | Cons | Reef recommendation |
|---|---|---|---|---|
| 35 ppt calibration solution | Calibrating where reefs operate | Most accurate at 35 ppt / 1.026 SG | Requires buying solution | Preferred for reef tanks |
| RO/DI (0 ppt) | Rinsing, quick zero check | Easy to access | Can be inaccurate at 35 ppt due to non-linearity | Use for cleaning/checks, not primary calibration |
Quick recap checklist (printable-style)
Calibration checklist
- Refractometer + 35 ppt solution at room temp (15–30 min)
- Prism and cover plate cleaned (no residue)
- 2–3 drops of 35 ppt solution, no bubbles
- Wait 30–60 seconds
- Focus scale, adjust to 35 ppt (or 1.026 SG)
- Repeat once to confirm
- Rinse with RO/DI and dry
Measurement checklist
- Sample from consistent location (avoid surface film)
- Enough liquid to fully cover prism
- Wait 30–60 seconds
- Read boundary line clearly
- Log ppt/SG and note top-off or water changes
FAQ: Calibrating a refractometer for reef tank salinity
Should I calibrate a refractometer with 35 ppt solution or RO/DI water?
For reef aquariums, calibrate with 35 ppt calibration solution. RO/DI water is great for rinsing, but calibrating at 0 can leave you off at reef salinity.
What is the correct salinity for a reef tank—ppt or specific gravity?
Either works if you’re consistent. A common reef target is 35 ppt, which is about 1.026 SG.
How often should I calibrate my reef tank refractometer?
For beginners, weekly is a safe habit. Calibrate immediately after drops/impacts, big temperature changes, or any time readings seem “off.”
Why does my refractometer show different readings each time?
Most inconsistencies come from dirty prisms, bubbles, too little sample, or temperature mismatch. Clean, use enough fluid, wait 30–60 seconds, and re-check calibration at 35 ppt.
Does ATC mean I don’t need to match temperatures?
No. ATC helps, but it doesn’t fully correct for large differences between the refractometer, calibration fluid, and sample water. Let items reach similar room temperature for best accuracy.
How long should sample water sit on the prism before reading?
Wait 30–60 seconds. This improves stability and reduces “wandering” lines.
Call to action: make salinity tracking easier
Once your refractometer is calibrated, the next win is consistency. Log salinity (ppt or SG), top-off events, and water changes so you can catch trends before animals react.
If you want a simple place to track readings and spot patterns, try Reef Buddy. And if you get stuck, Shrimpy can help you interpret your salinity logs and troubleshoot common refractometer errors in a beginner-friendly way.