Can I use table salt, rock salt, or sea salt instead?

No. Table salt contains anti-caking agents (sodium ferrocyanide, magnesium carbonate) and often iodine that harm fish. Rock and sea salts can contain iodine and trace heavy metals from their source, and their calcium/magnesium ratios are not optimised for aquarium use. Only aquarium-grade salt products should enter a tank — marine salt mixes (Instant Ocean, Tropic Marin, Red Sea) for brackish and marine setups, or pure aquarium salt (API, Seachem) for short-term freshwater treatments. The cost difference is small and the fish health difference is large.

How accurate do I need to be with specific gravity?

Within ±0.002 SG of target is fine for most brackish species. Within ±0.005 is where individual species tolerances start to become stressed. Fish acclimate to stable SG better than they adjust to fluctuating SG, so consistency over weeks matters more than hitting a specific number exactly. Use a refractometer (calibrated monthly with distilled water or a reference solution) rather than a hydrometer — hydrometers drift with age and temperature and routinely read 0.002-0.003 off.

Do brackish plants exist, and can I keep them?

Genuine brackish plants are uncommon in the hobby. Java fern and anubias tolerate SG up to about 1.003 with reduced growth. Vallisneria varieties do slightly better — V. americana is documented in naturally brackish habitat. Above SG 1.005, plant options narrow to true mangrove plantings (difficult for most home setups) or synthetic décor. The aquascaping-quality planted brackish tank is a niche with specialist supplier dependency. For most brackish community tanks, plan for hardscape-dominated aquascaping with minimal live plants.

How do I change a tank from freshwater to brackish?

Slowly — over 2-4 weeks — with partial water changes. Replace 15-20% of water weekly with water at 0.001-0.002 SG higher than current. Test SG weekly to confirm the slow rise. Rapid salinity changes (adding the full salt dose in one change) stress osmoregulation severely and can kill fish within hours. The same logic applies in reverse when returning a tank to freshwater. Coordinate the shift with a disciplined water change routine so the SG adjustment compounds cleanly into existing maintenance rather than adding a second intervention.

Aquarium Salt Mix Calculator — Brackish & SG

The Aquarium Salt Mix Calculator computes salt mass for target specific gravity, water change salt requirements, and monthly cost for brackish and low-end marine aquariums, with species-based SG guidance and a USD/GBP toggle.

Brackish Is Precise, Not Approximate

Brackish water is not "a bit salty". The specific gravity your fish need is precise, and eyeballing salt quantity is the single most common brackish-keeping failure. A freshwater community tank with a handful of salt added is not a brackish tank — it is a stressed freshwater tank where the molly is the only species benefiting and every other fish is slowly adapting to unwanted salinity. True brackish keeping requires measuring salinity with a calibrated refractometer, matching the target SG for your chosen species, and maintaining that SG across water changes.

The specific gravity range that matters for brackish species spans SG 1.002 to 1.010 — roughly 3 g/L to 13 g/L of salt by mass. Low brackish (1.002-1.006) suits mollies, Indian glassfish, and many hard-water livebearers. Mid brackish (1.006-1.010) is figure-8 puffer, bumblebee goby, and knight goby territory. High brackish (1.010-1.015) approaches marine and supports mudskippers and archerfish. Above SG 1.015, you are keeping marine fish, with the equipment and species selection implications that follow.

The Salt Mass Formula

For marine salt mixes, the empirical rule is 35 g/L to reach natural seawater (SG 1.026). This maps cleanly onto brackish ranges: grams per litre ≈ (target SG − 1.000) × 1333. At SG 1.005 (low brackish), that is 6.7 g/L; at SG 1.010 (mid brackish), 13.3 g/L; at SG 1.020 (soft marine), 26.7 g/L. The linear relationship holds across the brackish and low-marine range and breaks down slightly approaching full marine saturation, where manufacturer-specific mixing tables take over.

Total salt for a full tank fill is the per-litre mass times the net tank volume. A 100 L tank at SG 1.005 needs 670 g — about two-thirds of a kilogram of marine salt, which covers roughly 9-10 water changes before exhaustion. Per-water-change salt is the same per-litre rate applied to the change volume, so a 25% weekly water change on the same tank uses 6.7 × 25 = 168 g per week.

Marine Salt vs Aquarium Salt — Not Interchangeable

Marine salt mixes (Instant Ocean, Tropic Marin, Red Sea, Seachem Reef Salt) contain sodium chloride plus calcium, magnesium, potassium, sulphate, and trace elements — a chemical approximation of natural seawater. These are the correct salt for any brackish or marine target. Aquarium salt (API Aquarium Salt, Seachem Aquarium Salt) is sodium chloride only, manufactured as a short-term disinfecting treatment for freshwater fish — it lacks the calcium, magnesium, and trace elements that brackish and marine species need for osmoregulation and shell/skeleton formation.

Using aquarium salt in a brackish setup is the most common beginner mistake. Fish may appear healthy for weeks on the raised sodium, but chronic calcium and magnesium deficiency produces lateral-line disease, skeletal deformities, and reproductive failure over months. The calculator flags aquarium salt as unsuitable for any target SG above 1.003 and suggests switching to a marine mix. The cost difference is small — Instant Ocean runs about $4.50/kg, aquarium salt about $3.20/kg — and the fish health difference is substantial.

Never use table salt, rock salt, or sea salt from human-food sources. Anti-caking agents (sodium ferrocyanide), iodine additives, and trace heavy metal contamination in non-aquarium products cause acute and chronic fish health problems. Only aquarium-grade products should enter a tank.

Species-Specific SG Targets

Matching SG to the fish is the defining brackish-keeping discipline. The species database on this site includes SG ranges for the most common brackish fish: molly 1.002-1.005, Indian glassfish 1.002-1.006, bumblebee goby 1.005-1.010, knight goby 1.005-1.010, figure-8 puffer 1.005-1.008. Selecting an SG in the middle of the species range gives the most tolerance for natural drift between water changes.

Community brackish tanks are possible but require overlapping SG ranges. A molly-plus-Indian-glassfish community works well at SG 1.003-1.005, the overlap of both species ranges. A figure-8 puffer cannot share a tank with either species because of temperament (fin-nipping), not SG — most brackish puffers are species-only regardless of their salinity compatibility with other brackish fish. The stocking plan logic for compatibility applies normally; brackish just adds an SG-overlap requirement on top.

Water Change Discipline

Brackish water changes demand tighter discipline than freshwater. The replacement water must match the tank SG within ±0.002, and temperature within 2 °C. A 25% weekly water change in a 100 L tank at SG 1.005 means 25 L of premixed salt water at matched SG ready to go — not a freshwater top-up followed by salt addition.

The correct procedure is: premix salt in a separate container 24 hours before the change, allow it to dissolve fully with a powerhead or vigorous stirring, test the SG at the intended use temperature with a refractometer, adjust if needed, warm to tank temperature, then perform the water change. Adding dry salt directly to the tank is a disaster in the making — undissolved grains land on fish and substrate, localised salinity spikes kill inverts, and SG becomes impossible to verify until hours later. Coordinate brackish water changes with the overall water change routine so SG matching becomes part of the existing workflow rather than a separate task.

Equipment That Differs From Freshwater

Most brackish equipment is the same as freshwater — heaters, filters, lights all work identically. The exceptions are metal parts and stainless steel components that corrode faster in brackish water. Aluminium tank trim lifts/corrodes, cheap thermometer probes fail earlier, and any metallic décor (pirate ships with metal cannons, cheap plastic plants with wire reinforcement) releases heavy metals as the coating degrades. Plan a brackish setup with all-plastic, all-glass, or all-ceramic décor and inspect metal trim for oxidation annually.

The brackish tank also needs a tank volume calculation off the dimensioned input — every downstream sizing depends on net volume, and the salt mass scales off that same number. A filter flow for brackish bioloads sizes the same way as freshwater at 4-6× turnover per hour, and heater sizing for warm brackish species follows the same wattage-per-litre rules because most brackish species are tropical.

Costs and Storage

Marine salt retails for $4-5/kg in 5-10 kg buckets (Instant Ocean 10 kg ≈ $45). A typical low-brackish community tank uses 0.2-0.4 kg of salt per month for water changes — roughly $1.50-2 USD monthly (around £1.20-1.60 at the GBP toggle rate). Mid-brackish puffer setups run higher, closer to $3-4 USD monthly. This is cheap relative to fish food and electricity costs; brackish setups are not ongoing expensive to maintain once the equipment is in place.

Store marine salt in airtight containers — it is hygroscopic and absorbs moisture from humid air, which clumps it and affects mixing accuracy. A 5 kg bucket with a loose lid sitting in a garage for 18 months produces clumped, partially-oxidised salt that mixes unevenly. Transfer bulk purchases to lidded plastic bins with desiccant packets for long storage.

Brackish keeping is a niche within the freshwater hobby — most local fish shops stock only a few brackish species and may not have staff experienced with SG matching. Source fish from specialist brackish retailers or confirm SG acclimation history before buying. The reward is keeping some of the most characterful freshwater-adjacent species in the hobby — figure-8 puffers in particular are often cited as having the most recognisable personality of any common home-aquarium fish. Related discipline around precise dosing and environmental matching appears across pet care — precision-dosed feline medication uses the same mindset of "close enough is dangerous", and the signs of mismatched parameters surface early through the kind of behavioural vigilance covered in the recognising animal stress from environmental shifts guide.

Aquarium Salt Mix Calculator