How much salt per gallon?
For a reef tank at 35 ppt (specific gravity 1.026), you need about 135 grams of salt mix for every US gallon of fresh water. That is roughly half a cup. In metric it is about 36 grams per liter.
Worked example. You are mixing 5 gallons (18.9 liters) for a reef tank at 35 ppt. 18.9 × 35 × 1.02 = 675 grams, about 1.5 pounds or 2.4 cups.
Treat the answer as a starting point. Brands of salt mix differ by a few percent, and salt soaks up moisture from the air once the bag is open. Add most of the salt, mix until the water is clear, measure, and then add the rest a little at a time.
Salinity and specific gravity
There are two ways to say how salty water is.
- Salinity (ppt) is parts per thousand: grams of salt in a kilogram of water. Sea water is about 35 ppt.
- Specific gravity (SG) is how heavy the water is compared with fresh water. Sea water is about 1.026.
They measure the same thing. This table converts between them at 77 °F (25 °C):
| Tank | Salinity (ppt) | Specific gravity |
|---|---|---|
| Brackish (mollies, bumblebee gobies) | 4–12 | 1.003–1.009 |
| Fish-only marine | 29–33 | 1.022–1.025 |
| Reef, with corals | 34–36 | 1.025–1.027 |
| Natural sea water | 35 | 1.026 |
How to mix salt water
- Fill a clean bucket with fresh water. Reef keepers use RO/DI water, which has had everything else filtered out.
- Put a small pump or powerhead in the bucket, and a heater if the room is cool.
- Add the salt to the water, never water to salt.
- Let it mix until the water is completely clear. Many mixes need a few hours.
- Measure with a refractometer. Adjust: more salt if it is low, more fresh water if it is high.
- Match the temperature to the tank before you use it.
This is not "aquarium salt" for freshwater tanks
Marine salt mix contains the full blend of minerals found in sea water. The plain "aquarium salt" sold for freshwater fish is just sodium chloride, used in small doses as a treatment. They are different products for different jobs. Never use table salt with anti-caking agents in any aquarium.
Common mistakes
- Mixing salt in the tank. Undissolved grains burn fish and corals. Always mix in a separate bucket.
- Topping up with salt water. Only water evaporates; the salt stays behind. Top up with fresh water, or the tank gets saltier every week.
- Trusting a cheap swing-arm hydrometer. They read wrongly when bubbles cling to the arm. A refractometer is far more reliable.
- Measuring by cups alone. A cup of salt can weigh very different amounts from brand to brand. Weigh it, then check with a refractometer.
Questions and answers
How much salt do I need per gallon for a saltwater tank?
About 135 grams, or roughly half a cup, of marine salt mix per US gallon gives 35 ppt (specific gravity 1.026). Check the result with a refractometer, because brands differ.
How much salt for 5 gallons of water?
About 675 grams (1.5 pounds, or roughly 2.4 cups) of marine salt mix for a reef tank at 35 ppt.
What should the salinity of a reef tank be?
35 ppt, which is a specific gravity of 1.026 at 77 °F (25 °C). Keep it steady: corals dislike swings more than a slightly high or low number.
How do I convert specific gravity to ppt?
Subtract 1, then divide by 0.00075. For example 1.026: 0.026 ÷ 0.00075 is about 35 ppt. This holds at about 77 °F (25 °C).
How do I lower salinity that is too high?
Take some tank water out and replace it with fresh RO or dechlorinated water, a little at a time over several hours. Never drop salinity fast.
Sources
- Sea water at 35 ppt has a specific gravity of about 1.0264 at 25 °C (UNESCO equation of state for seawater, EOS-80)
- Salt mix makers' directions: about half a cup per US gallon for 1.025–1.026; verify with a refractometer