Aquarium salt mix calculator

Every salt maker publishes a dose: so many grams per litre gives a stated salinity. Their own charts scale that dose in straight proportion, so the calculator does the same. The salt you need is the dose per ppt, multiplied by how far the salinity has to rise, multiplied by the volume.

That one rule covers both jobs. For a fresh mix the salinity rises from 0, so 100 litres of Red Sea Coral Pro at 35 ppt needs 3.82 kg. For a mix that came out at 33 ppt it only has to rise by 2, so the same 100 litres needs about 218 g more.

Salt mixes are not pure dry salt. They differ in moisture and in how much of their weight is water bound into the crystals, so the weight needed to reach the same salinity differs from brand to brand. Scaled to 35 ppt, Aquaforest's published dose comes to about 414 g per 10 litres against 382 g for Red Sea Coral Pro, a difference of around 8%. Across the eight salts listed, the dose for 35 ppt runs from about 37.5 g per litre (Brightwell NeoMarine) to 42 g (Instant Ocean Reef Crystals).

Only salts whose maker publishes a weight per litre against a salinity are listed. If yours is not there, choose the last option and enter the figure from its packet. If you do not know your salt's figure, the typical setting will get you close, and the refractometer does the rest.

Salinity is the grams of dissolved salt in a kilogram of seawater, written as ppt (parts per thousand). Specific gravity (SG) is how dense the water is compared with pure water. Both describe the same thing, and the calculator converts between them at 25 °C, the temperature refractometers are calibrated to. Natural seawater is 35 ppt, which reads SG 1.026.

Most reef keepers aim for 35 ppt, or a little lower at 33 to 34. Whatever you choose, keep it steady: matching every water change to the tank matters more than the exact number. Calibrate your refractometer with 35 ppt calibration fluid rather than fresh water, because a small error at zero grows at full strength.

1. Fill a clean container, kept only for salt water, with RO/DI water. Tap water can bring in phosphate, nitrate and metals.
2. Bring the water to your tank's temperature with a heater before you add the salt.
3. Add the salt to the water, never the water to the salt, with a pump running to keep it moving.
4. Mix until the water is clear, following your salt's own mixing time. Some salts are ready in under an hour, and some, such as Red Sea Coral Pro, should not be kept mixed for more than 12 hours.
5. Check the salinity with a calibrated refractometer, then add a little salt or a little RO/DI water to fine-tune it.