Salinity Calculator
Calculate exactly how much salt mix you need for your marine, reef, or brackish aquarium. Includes brand comparison and mixing instructions.
Salt Mix Brand Comparison
| Brand | Amount Needed | Best For |
|---|---|---|
| Instant Ocean | 10.0 cups (2,730 g) | Most popular, widely available, great for FOWLR |
| Red Sea Coral Pro | 10.0 cups (2,730 g) | Higher calcium/alkalinity, ideal for reef tanks |
| Fritz RPM | 10.0 cups (2,730 g) | Consistent batch quality, fast dissolving |
Mixing Instructions
- Always mix salt into RO/DI water in a separate container, never directly into the tank
- Mix with a powerhead for at least 30 minutes until fully dissolved
- Let the water reach target temperature before adding to the tank
- Verify salinity with a refractometer before use (not a hydrometer)
- Adjust salinity gradually -- no more than 1-2 ppt change per day for livestock safety
How It Works
This calculator determines how much salt mix to add based on your tank volume, current salinity, and target salinity. The formula scales proportionally: approximately 1/2 cup of salt mix per gallon of water produces full marine salinity (35 ppt).
For partial changes, the formula calculates only the amount needed to bridge the gap between current and target salinity. For example, going from 30 ppt to 35 ppt requires much less salt than mixing from fresh water.
Specific gravity (SG) is an alternative way to measure salinity and is commonly used by hobbyists. SG of 1.025 corresponds to approximately 35 ppt at 77F (25C). Temperature affects SG readings, so refractometers are preferred over hydrometers.
All major salt mix brands (Instant Ocean, Red Sea, Fritz) use approximately the same ratio of salt to water. The differences are in trace element concentrations, which matter mainly for reef tanks with demanding corals.
Frequently Asked Questions
What salinity should a saltwater aquarium be?
A marine fish-only tank should be kept at 33-35 ppt (specific gravity 1.023-1.025). Reef tanks with corals are best at 35 ppt (SG 1.025-1.026). Brackish tanks vary widely: low brackish is 5-10 ppt, mid-brackish is 10-18 ppt, and high brackish is 18-25 ppt. Consistency is more important than hitting an exact number -- avoid fluctuations of more than 1 ppt per day.
How do I measure salinity accurately?
A refractometer is the most accurate and affordable tool for measuring salinity. Calibrate it with RO/DI water or a 35 ppt calibration solution before each use. Hydrometers (swing-arm or floating) are less accurate and prone to salt buildup that skews readings. Digital salinity monitors are the most convenient but cost significantly more.
Can I use table salt instead of aquarium salt mix?
No. Table salt contains anti-caking agents and sometimes iodine, which can harm marine invertebrates. Aquarium salt mixes like Instant Ocean, Red Sea, and Fritz contain a precise blend of major and trace elements (calcium, magnesium, alkalinity, strontium, etc.) that replicate natural seawater. These elements are essential for coral and invertebrate health.
How often should I check salinity in my saltwater tank?
Check salinity at least weekly, and before and after every water change. Evaporation removes water but not salt, so salinity naturally rises between top-offs. Use an auto-top-off (ATO) system with RO/DI water to maintain stable salinity. Always replace evaporated water with fresh RO/DI water, not saltwater.