Aquarium Salt Mix Calculator — Marine & Freshwater

Aquarium Salt Guide

What do I need to know about Marine Salinity Standards?

Reef aquariums: 35 ppt (parts per thousand) = specific gravity 1.025 at 25°C. This matches natural ocean water. Coral and invertebrates thrive at this level. Fish-only marine: 30-32 ppt (SG 1.020-1.022). Slightly lower than natural ocean — reduces parasite reproduction rates. Salt cost savings. Hyposalinity treatment: 14 ppt (SG 1.009-1.010). Therapeutic level — eliminates marine ich (Cryptocaryon irritans) without harming fish. Must be maintained for 6-8 weeks minimum. Not suitable with invertebrates present, since the treatment level of salinity used to eliminate ich is toxic to shrimp, snails, and other invertebrates in the same tank.

What do I need to know about Mixing Marine Salt Correctly?

Use RO/DI water — never tap water. Tap water contains chlorine, copper, nitrates, phosphates. Heat water to 25°C before adding salt. Add salt slowly to water (never water to salt). Mix vigorously with a powerhead or large stir paddle for 12-24 hours. Aerate to drive off CO2 — raises pH. Check salinity after 24 hours with refractometer (NOT a hydrometer — refractometers are far more accurate). Refractometer calibration: zero with RO water, check with calibration fluid (53ppt) periodically. Inaccurate hydrometers are a common source of stocking and dosing errors, which is why serious reef keepers invest in a proper refractometer instead.

What should I know about Water Changes?

Standard reef tank: 10-20% weekly water change. Removes accumulated nitrates, phosphates, replenishes trace elements depleted by corals. New water must match tank: same salinity, similar temperature (±1°C), pH within 0.2. Premixed water in a separate brute can with powerhead — mix 24 hours before use. Acclimating fish to new salinity: never change more than 0.001 SG per hour. Faster changes cause osmotic shock and fish death. Drip acclimation: tube from tank to bag with an airline valve regulated to a slow drip, gradually mixing tank water into the transport bag over 30-60 minutes to acclimate new fish safely.

What's the key thing to understand about Freshwater Salt — Therapeutic?

Aquarium salt (sodium chloride only — NOT marine salt) used therapeutically in freshwater. Doses: low dose: 1g/litre. Useful for ich treatment, slime coat support, mild bacterial infections. Most fish tolerate this indefinitely. Medium dose: 2g/litre. More aggressive ich treatment. Many species (livebearers, cichlids) tolerate. Catfish, scaleless species, plants suffer. High dose: 3g/litre. Strong treatment but limited duration (10-14 days). Many sensitive species cannot tolerate. CAUTION: salt does not evaporate with water changes, so repeated partial water changes without accounting for this will gradually concentrate salt to dangerous levels if not carefully tracked.

Why Must Marine Salt Mix Be Fully Dissolved and Temperature-Matched Before Adding to a Tank?

Marine salt mix needs time to fully dissolve and for the water's pH and alkalinity to stabilise after mixing, which is why experienced marine aquarists typically mix new saltwater in a separate container and let it circulate for several hours to overnight before adding it to the main tank, rather than mixing and adding immediately — freshly mixed saltwater often has an unstable, elevated pH that settles over this waiting period as the salt fully dissolves and the water equilibrates with air. Adding water to the tank that is not temperature-matched to the existing tank water, even if correctly salted, can also cause thermal shock to sensitive marine fish and invertebrates, which is why mixed replacement water is generally brought to match the tank's temperature before use, not just checked for correct salinity, since a correctly salted but temperature-mismatched water change can be just as harmful as an incorrect salinity level.

Why Does Salinity Need to Be More Precisely Controlled in Reef Tanks Than Fish-Only Marine Tanks?

Fish generally tolerate a somewhat wider range of salinity fluctuation than many reef invertebrates do — corals, in particular, are highly sensitive to even small, rapid changes in salinity, since their biological processes (including the symbiotic algae many corals depend on) are adapted to the very narrow and stable salinity range of natural reef environments. This is why reef tank keepers generally maintain tighter salinity targets and more frequent, careful monitoring (specific gravity typically targeted around 1.023-1.026, checked with a precise refractometer rather than a less accurate hydrometer) compared with fish-only marine setups, where a slightly wider acceptable salinity range is generally tolerated without issue. A rapid, significant salinity swing from a mis-measured water change or a salt-mixing error is a well-documented cause of coral stress and bleaching events in reef aquariums, which is part of why precise salinity measurement tools and careful water-change technique are considered essential rather than optional for anyone keeping corals or other sensitive reef invertebrates.

What should I know about Refractometers vs Hydrometers for Measuring Salinity?

Two common tools measure aquarium salinity, with meaningfully different accuracy. A swing-arm hydrometer is inexpensive and simple to use but has a wider margin of error and can drift out of calibration or stick over time, making it a reasonable budget option for basic fish-only marine setups where some imprecision is tolerable. A refractometer measures salinity using light refraction and offers significantly better precision, which matters more as a tank's stocking becomes more sensitive to salinity swings — reef tanks with corals and invertebrates generally warrant the better accuracy a refractometer provides, since many reef inhabitants are considerably less tolerant of salinity fluctuation than typical fish-only setups. Regardless of which tool is used, periodic calibration against a known reference solution helps catch drift before it leads to an inaccurate reading being trusted during a water change.

Specific Gravity and Salinity Reference Table

Marine salinity is measured either as specific gravity (SG, a ratio compared with pure water) or as parts per thousand (ppt) salt content; both describe the same thing and are read with a hydrometer or, more accurately, a refractometer.

Readings taken with a hydrometer are accurate to roughly +/- 0.001-0.002 SG and are temperature-sensitive, which is enough error to matter for sensitive reef invertebrates — a calibrated refractometer, checked periodically against a known reference solution, is the more reliable tool for reef tanks.

Worked Example: Mixing Salt for a Water Change

To prepare 20 litres of new saltwater for a reef tank targeting 1.025 SG (about 35ppt), using a marine salt mix that specifies roughly 35g of salt per litre to reach that salinity: 20 litres x 35g/litre = 700g of salt mix dissolved into 20 litres of dechlorinated, temperature-matched RO or treated tap water. The mix should be aerated and circulated for at least several hours (many reefers mix overnight) until fully dissolved and the specific gravity has stabilised, since undissolved salt continues to raise the reading as it dissolves, and adding water that is not yet temperature-matched to the main tank can shock fish and corals during the change.

Common Mistakes When Adjusting Salinity

The two most frequent errors in aquarium salt dosing are adding dry salt mix directly into the tank (rather than fully dissolving it in a separate container first), which can cause localised salinity spikes and burns on fish gills and coral tissue, and changing salinity too quickly. Fish and invertebrates osmoregulate (balance internal salt and water content) and need time to adapt; a swing of more than about 0.001-0.002 SG per hour, or more than roughly 10% of tank volume changed at once with water of a different salinity, is considered too abrupt for a fully stocked tank and can stress or kill sensitive species. Gradual drip acclimation is standard practice when introducing new livestock from water of a different salinity than the main tank.

Ramping Salinity for a Quarantine Tank

Setting up a quarantine tank for new marine fish sometimes calls for adjusting salinity gradually rather than matching the main display tank exactly, particularly when using hyposalinity as part of a parasite treatment protocol (lower salinity stresses many external parasites more than it stresses the fish). A typical hyposalinity protocol lowers specific gravity from a normal reef range of around 1.025 down to roughly 1.009-1.010 over 24-48 hours, held there for the treatment duration (commonly 2-4 weeks, depending on the specific parasite and protocol being followed), then raised back toward normal just as gradually at the end. Rapid salinity swings in either direction stress fish osmotically, so both the drop into hyposalinity and the return to normal salinity should happen incrementally rather than in one step. This is a targeted treatment technique best researched thoroughly or guided by an experienced aquarist for the specific condition being treated, rather than attempted casually, since getting the ramp rate or duration wrong can harm the fish it's meant to help.