Mixing Saltwater for Reef Aquariums
Creating synthetic seawater is the foundation of the marine aquarium hobby. While natural seawater is a complex soup of minerals and trace elements, modern marine salt mixes are engineered to replicate those conditions perfectly. However, achieving the correct salinity is not just about dumping salt into a tank; it requires precise volumetric calculation and an understanding of how density changes with concentration.
Salinity vs. Specific Gravity
In the reef hobby, you will see two measurements used interchangeably: Salinity (measured in parts per thousand, or ppt) and Specific Gravity (SG). Salinity is an absolute measure of the total mass of dissolved salts in the water. Specific Gravity is a relative measure of density. At a standard reef temperature of 77°F (25°C), a salinity of 35 ppt equals a specific gravity of 1.0264.
How Much Salt per Gallon?
Most commercial salt mixes are designed so that approximately 1/2 cup of salt per US gallon will yield a salinity of roughly 30-32 ppt. To reach the standard reef target of 35 ppt, you generally need about 0.55 to 0.6 cups per gallon. This calculator uses a precise gram-based mass approach, which is far more accurate than volumetric "cups" because different salt brands have different grain sizes and densities.
The Math Behind the Mix
The calculation follows the direct proportional relationship of mass per volume. For a target of 35 ppt, we assume a requirement of 35 grams of dry salt mix for every 1000 grams (1 liter) of water. While salt mixes contain some moisture, this 1:1 ratio is the gold standard for starting a batch of water.
Best Practices for Mixing
- RO/DI Water Only: Never use tap water for a reef tank. The minerals and chemicals in tap water interact with the salt mix to create unpredictable and often toxic results.
- Add Salt to Water: Always fill your mixing container with water first, then add the salt slowly while a powerhead provides circulation. Adding water to a pile of salt can cause a localized "super-saturation" that results in calcium precipitation (white "snow" that won't dissolve).
- Wait 24 Hours: Freshly mixed saltwater is chemically aggressive. Let it circulate for at least 12-24 hours to ensure the pH and oxygen levels have stabilized before adding it to your aquarium.
Frequently Asked Questions
Why is my salinity higher the next day?
This is usually due to evaporation. As pure water evaporates from your mixing bucket, the salt stays behind, increasing the concentration. Always keep your mixing bucket covered or check the salinity right before doing your water change.
Does water temperature change the salt dosage?
The *amount* of salt needed doesn't change, but the *reading* on your hydrometer or refractometer will. Refractometers are typically ATC (Auto Temperature Correcting), but if you use a plastic swing-arm hydrometer, the reading will be wildly inaccurate if the water isn't at the calibration temperature.