Reef & Marine Calculators

Saltwater is less forgiving

Reef and marine tanks hold animals that evolved in a very stable environment. Salinity, alkalinity, calcium and magnesium all need to be kept inside narrow ranges, and large swings stress or kill corals and other invertebrates. The tools on this page help you size equipment, mix salt, convert salinity readings and correct parameters in small steps.

Which calculator answers which question

• How big a skimmer? The Protein Skimmer Size Calculator adjusts a rated capacity for system volume and bioload.
• How much to raise alkalinity, calcium or magnesium? The Reef Dosing Calculator uses common two-part ratios.
• What is my salinity? The Salinity SG to PPT Converter converts between specific gravity and parts per thousand.
• How much salt mix? The Salt Mix Salinity Calculator works out salt for a volume.

Salinity: 35 ppt and its readings

Natural seawater is about 35 parts per thousand, and most reef tanks run at about 1.025 to 1.026 specific gravity, roughly 33 to 35 ppt, depending on the temperature at which the hydrometer is calibrated. A hydrometer is the cheapest tool and the least accurate. A refractometer calibrated with a reference fluid, or a calibrated probe, is more reliable, and the converter is a good approximation but is not a substitute for either.

Skimmers and system volume

A protein skimmer processes whatever water passes through it, which in a typical reef includes the sump, so the sizing basis should be the total system volume and not just the display. Manufacturer ratings usually assume an average mixed-reef load, so a heavily fed or heavily stocked tank needs a larger skimmer, and a lightly stocked one may need less. Many reef keepers choose a skimmer a little over the minimum and run it at a well-tuned setting.

Dosing: small steps, then test

Corals take up alkalinity, calcium and magnesium, and keepers replace them by dosing. The calculator uses commonly published ratios for homemade two-part recipes, which differ from commercial products, so always check your product's label. Large corrections in one step are dangerous, so make changes gradually, test before and after with a reliable kit and keep alkalinity stable rather than chasing a number.

Mixing salt

Salt mixes differ by brand, so the amount per gallon is an input and not a constant. Mix in a separate container with a heater and a pump, give it time to dissolve and stabilize, check salinity and temperature and match the new water to the tank before you use it for a water change. The cup conversion is only approximate, because salt packs differently, so weigh when you can.

Common mistakes

• Trusting a hydrometer alone. Calibrate or use a refractometer.
• Dosing from a recipe without checking the label. Products differ in strength.
• Correcting too fast. Swings harm corals more than slightly off numbers.
• Sizing a skimmer on display volume only. Include the sump.