Calculator Form
Example Data Table
| Scenario | Volume | Current | Target | Expected Action |
|---|---|---|---|---|
| Reef tank increase | 100 L | 33 ppt | 35 ppt | Add about 200 g dry salt before purity adjustment. |
| Marine dilution | 75 L | 37 ppt | 35 ppt | Replace about 4.05 L with fresh water. |
| Brackish setup | 120 L | 5 ppt | 8 ppt | Add about 360 g dry salt before purity adjustment. |
Formula Used
Salinity conversion: 1 ppt is treated as about 1 g/L for aquarium planning. PSU and g/L are treated as close planning equivalents.
Specific gravity estimate: salinity ppt = (corrected SG - 1) / 0.000742. Corrected SG = reading + ((sample temperature - calibration temperature) x 0.0002).
Dry salt dose: salt grams = (target ppt - current ppt) x tank liters / purity fraction.
Freshwater dilution: freshwater liters = tank liters x (1 - target ppt / current ppt).
Replacement water mixing: change liters = tank liters x (target ppt - current ppt) / (replacement ppt - current ppt).
Evaporation estimate: new salinity = current ppt x tank liters / remaining liters.
How to Use This Calculator
- Enter the aquarium volume and select the correct unit.
- Enter the current salinity reading from your instrument.
- Select the reading type, such as SG, ppt, PSU, ppm, percent, or g/L.
- Enter the target salinity and matching target unit.
- Add temperature details when using specific gravity readings.
- Enter replacement water salinity if planning a water change.
- Press calculate, then review the result above the form.
- Download the CSV or PDF report when records are needed.
Understanding Aquarium Water Salinity
Aquarium salinity describes the dissolved salt held in water. It is a mass ratio, so physics makes it easier to estimate. One part per thousand means one gram of dissolved salt in one liter of water. Marine aquariums often sit near natural seawater. Many reef systems use about 35 ppt. Brackish tanks use lower values. Freshwater systems should stay close to zero.
Why Stable Salinity Matters
Fish, corals, shrimp, and beneficial bacteria balance water through osmosis. A sudden salinity change can pull water from cells. It can also force water into cells. Both conditions stress livestock. Stable readings support respiration, feeding, waste control, and mineral balance. Salinity also affects density. Denser water gives higher specific gravity readings. That is why hydrometers and refractometers need calibration.
How This Calculator Helps
This calculator converts common salinity readings into ppt. It accepts specific gravity, PSU, percent, ppm, and grams per liter. It estimates salt mass needed when salinity is low. It also estimates freshwater replacement when salinity is high. The water change option uses a mixing equation. It can plan changes with stronger or weaker replacement water. The staged change value helps avoid large shifts.
Practical Measurement Tips
Always test water at a known temperature. Rinse refractometer glass after each use. Calibrate with proper fluid, not random tap water. Mix new saltwater before adding it. Let the mixture clear and reach tank temperature. Check salinity again before use. Evaporation raises salinity because salt stays behind. Top off evaporated water with fresh water only. Do not top off with saltwater unless correcting a known mistake.
Safe Use Notes
The results are planning estimates. Aquarium salt blends vary by brand and moisture level. Actual dosing may need small adjustments. Add only part of the calculated salt first. Retest after full mixing. Large corrections should be split over several stages. Sensitive reef animals need slow changes. Record every reading, dose, and water change. Good records make future maintenance safer and easier.
Common Target Ranges
Reef aquariums often use 34 to 36 ppt. Fish only marine tanks may use slightly lower values. Brackish aquariums vary widely. Match the species, not a guess. Change targets slowly when livestock is already present for best results.
FAQs
What is aquarium salinity?
Aquarium salinity is the amount of dissolved salt in water. It is often shown as ppt, PSU, specific gravity, ppm, percent, or grams per liter.
Is ppt the same as PSU?
They are not perfectly identical in scientific work. For aquarium planning, they are commonly treated as close enough for routine salinity adjustment.
How much salt raises salinity by 1 ppt?
About one gram of salt raises one liter of water by 1 ppt. Salt blend moisture and impurities can change the real amount.
Why does evaporation raise salinity?
Evaporation removes water but leaves salt behind. The same salt mass remains in less water, so the concentration rises.
Should I top off with saltwater?
Usually no. Normal evaporation should be replaced with fresh water. Use saltwater only when correcting a measured salinity problem.
Can this calculator replace testing?
No. It gives planning estimates. Always test with a calibrated refractometer, hydrometer, or meter before and after any correction.
What salinity is best for reef tanks?
Many reef aquariums target about 35 ppt. Some keepers use a slightly different range. Match livestock needs and keep readings stable.
Why split large salinity corrections?
Fast salinity changes can stress fish, corals, shrimp, and bacteria. Staging changes gives livestock time to adjust through osmosis.