Aquarium Salt Calculator — Marine, Brackish & Hospital Dosing
Calculate exact marine salt quantities for your tank volume and target salinity. Works for reef (SG 1.025), FOWLR, brackish, and freshwater hospital salt treatments.
Calculator
How to use the salt calculator
- Enter tank volume — US gallons of actual water (tank minus rock, sand, equipment).
- Select target salinity — Reef (1.025 SG), FOWLR (1.023), brackish (choose level), or hospital dosing.
- Read salt weight — Output in pounds and grams with tolerance for brand variation.
- Mix in separate container — Never add dry salt to a tank with fish — mix in a bucket with RO/DI and aerate 24 h first.
Salinity targets by tank type
Specific gravity (SG) and salinity in parts per thousand (ppt) describe the same thing in different units — dissolved salts in water. Every saltwater-system type has a well-defined target:
| Tank type | Specific gravity | Salinity (ppt) | Salt per gallon |
|---|---|---|---|
| Reef / mixed reef | 1.025 | 35 | 0.50 lb |
| FOWLR (fish only + live rock) | 1.023 | 33 | 0.47 lb |
| High-brackish (mono, scat) | 1.018 | 24 | 0.35 lb |
| Mid-brackish (figure-8 puffer) | 1.010 | 13 | 0.20 lb |
| Low-brackish (molly, bumblebee goby) | 1.005 | 7 | 0.10 lb |
| Hospital tank ich treatment | 1.002 | 3 | 1 tbsp / 3 gal |
How to mix saltwater correctly
Every experienced reefer follows the same mixing routine because shortcuts hurt the tank. The full protocol:
- Start with RO/DI water — tap water contains phosphates, silicates, and chloramine that feed algae and poison invertebrates. RO/DI home systems cost $150–$300; public refills are 25–50¢/gal.
- Heat to target temperature — 77–78 °F for reef. Cold water reads inaccurate on refractometers and dissolves salt unevenly.
- Add salt gradually — pour into a flowing powerhead, let one handful fully dissolve before adding more. Raw undissolved salt on the bottom corrodes plastic.
- Mix 24+ hours with powerhead — allows all salt components (calcium, magnesium, strontium, trace elements) to fully dissociate.
- Verify salinity with refractometer — calibrate to 35 ppt cal fluid monthly. Tweak with RO/DI (lower) or salt (raise) until within 0.0005 SG of target.
- Match to display temperature, pH, and salinity before use. Even 0.5 °F difference can shock inverts during water change.
Aquarium salt as a freshwater fish medicine
Aquarium salt (pure NaCl, not marine mix) is one of the oldest and most effective treatments for common freshwater fish diseases. Dosing:
- Ich (white-spot disease): 1 tbsp per 3 gal; raise temperature to 82 °F; treat 7–10 days. Salt disrupts the free-swimming tomite stage.
- Fin rot, minor bacterial infections: 1 tbsp per 5 gal for 7 days.
- Nitrite poisoning emergency: 1 tbsp per 10 gal blocks nitrite uptake at gills (chloride competes with nitrite). Lifesaver during accidental cycle crashes.
- Scaleless species (cory, pleco, tetra, loach): Half dose, monitor closely. Freshwater stingrays and mormyrids tolerate no salt — use other meds.
Keep salt-treated water at the treatment concentration during water changes — don't let it dilute. When done, phase out salt over 2–3 water changes rather than a sudden drop.
Frequently Asked Questions
How much marine salt per gallon for a reef tank?
Approximately 0.5 lb (227 g) of marine salt mix per US gallon reaches specific gravity 1.025 (35 ppt salinity) — the reef standard at 77 °F. Rates vary slightly by salt brand: Instant Ocean ~0.53 lb/gal, Red Sea Coral Pro ~0.51 lb/gal, Tropic Marin Pro ~0.50 lb/gal. Always verify final salinity with a refractometer — not a hydrometer (10–15 % inaccurate).
What's the right salinity for a brackish aquarium?
Brackish aquariums (mollies, bumblebee gobies, mono sebae, puffer fish) run 5–20 ppt (SG 1.003–1.015). Low-brackish (mollies): 1.005 SG = about 0.1 lb salt/gal. Mid-brackish (figure-8 puffers): 1.010 SG = 0.2 lb/gal. High-brackish / almost marine (monos, scats): 1.018 SG = 0.35 lb/gal. Pure freshwater is 0 ppt; full reef marine is 35 ppt.
Do I need a refractometer or is a hydrometer OK?
Use a refractometer. Swing-arm hydrometers have 10–15 % error that can drift further with buildup, leading to chronic under-salting (fish stress) or over-salting (coral death). Refractometers cost $20–$40 for a decent model (Milwaukee MA887 digital is $90 and temperature-compensated). Calibrate with 35 ppt calibration fluid every 2–3 months.
How often should I mix new saltwater?
Mix 24–48 hours before a water change to let salt fully dissolve and stabilize. Use RO/DI water (reef) or RO water (FOWLR). Heat to tank temperature, aerate with a powerhead, and verify salinity matches display tank within 0.001 SG. Mixing at the last minute causes pH spikes and incomplete dissolution that can shock sensitive invertebrates.
Can I use aquarium salt in a hospital tank for freshwater fish?
Yes — aquarium salt (not marine mix) at 1 tablespoon per 3 gallons treats ich, fin rot, and nitrite poisoning in most freshwater species. Scaleless fish (corydoras, loaches, plecos, African tetras) tolerate only half that rate (1 tbsp per 6 gal). Treatment length 7–14 days; replace salt with each water change to maintain concentration.
Why RO/DI water for saltwater tanks?
Tap water contains phosphates, silicates, nitrates, copper, and chloramine that fuel algae, poison inverts, and accumulate as salt mix water evaporates. RO/DI (reverse osmosis + deionization) strips 99 % of these. Cost: $150–$300 for a home RO/DI unit; refill water runs <5 cents per gallon. Most reef failures trace back to tap water use.
How do I adjust salinity if it's too high or too low?
Too high: swap tank water for RO/DI freshwater (no salt) at the same temperature. Volume to swap = tank vol × (current SG − target SG) / current SG. Too low: swap tank water for pre-mixed higher-salinity water. Never adjust more than 0.003 SG per day — rapid salinity swings kill inverts. Top-off with fresh RO/DI only (evaporation removes water, not salt).
Sources & References
- [1]Salt Mix Comparison and Mixing Best Practices — Bulk Reef Supply
- [2]Reef Aquarium Water Parameters — Reef Builders
- [3]Salt Treatments in Freshwater Aquariums — Aquarium Co-Op
- [4]Marine Salt Chemistry Fundamentals — Reef2Reef
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