Brackish Tank Marine Salt Mixing Protocol Guide: Specific Gravity Recipe

The single most common brackish setup mistake we see at the shop is using aquarium salt instead of marine salt. The two are not interchangeable — aquarium salt is sodium chloride only, while marine salt includes the trace minerals brackish ecosystems actually need. Get the brackish marine salt mixing protocol wrong and your livestock survives but never thrives, your plants yellow, and your water never reaches the right ionic balance. This guide from Gensou Aquascaping at 5 Everton Park covers the recipe, equipment and process to mix accurate brackish water from Singapore RODI baseline every time.

Marine Salt vs Aquarium Salt

Aquarium salt — sometimes labelled “tonic salt” or “rock salt” — is essentially purified sodium chloride with trace anti-caking agents. It treats minor freshwater issues but provides none of the calcium, magnesium, potassium and trace elements estuarine animals require. Marine salt blends like Tropic Marin Pro Reef and Red Sea Coral Pro replicate natural seawater ionic ratios and are the correct base for any brackish setup. Use them at full strength scaled to your target SG.

The Recipe

The conversion runs roughly linear at brackish concentrations: 8 g marine salt per litre of RODI water yields SG 1.005, 16 g/L yields SG 1.010, 24 g/L yields SG 1.015. For a sailfin molly tank at SG 1.010 with 100 litres total volume, you need 1.6 kg of marine salt. Always weigh on a kitchen scale; volume measurements with scoops drift wildly because salt clumps with humidity.

RODI Baseline

Singapore tap water already contains chloramine, dissolved minerals and varying chlorine levels — none of which you want as the foundation for precise salinity. Reverse-osmosis-deionised (RODI) water gives a 0 TDS starting point. Standard RODI units cost SGD 150-400 and process 80-150 litres per hour. Bottled distilled water from supermarkets works for small tanks under 30 litres but becomes uneconomical above that. Stock up on RODI capacity through the aquarium equipment range.

Mixing Container and Process

Mix in a separate clean food-grade container — never directly in the display tank. A 25-litre HDPE bucket from any hardware store works. Add salt slowly to RODI while stirring with a powerhead or air stone. Full dissolution takes 30-60 minutes; let the mix circulate for 24 hours before use to allow ionic equilibrium and gas exchange.

Refractometer vs Hydrometer

A swing-arm hydrometer is cheap (SGD 10-20) but inaccurate, often off by 0.002-0.004 SG. A handheld refractometer (SGD 30-80 from the aquarium equipment range) reads to 0.001 SG and is the standard for any serious brackish or marine work. Calibrate monthly with distilled water — the screen should read exactly 1.000 at 25°C. Replace cheap refractometers if calibration drifts.

Temperature Compensation

SG readings shift with temperature. Most refractometers display readings normalised to 20°C; aquarium water at 26-28°C reads slightly low. Automatic temperature compensation (ATC) refractometers handle this correction internally. Without ATC, expect about 0.001 SG difference between cold and warm samples — measurable but rarely critical for mild brackish.

Topping Up Evaporation

Salt does not evaporate; only water does. Top-ups for evaporation use plain RODI water, not premixed salt water. Mark the original waterline on the tank with tape. Top up to that line with RODI; salinity returns to target. Adding salt water during top-up causes salinity creep over weeks. The water care and treatment range stocks RODI-compatible conditioners that work in brackish chemistry.

Water Changes

Mix replacement water at the exact target SG, temperature-matched within 1°C. Weekly 20-25 per cent changes maintain trace mineral levels and remove nitrate. Brackish nitrification cycles slower than freshwater because some nitrifying bacteria are salinity-sensitive — start with smaller, more frequent changes (10 per cent twice weekly) for the first three months of a new brackish tank.

Common Mixing Mistakes

Three errors come up repeatedly. First, dumping salt directly into the display tank — this creates dangerous salinity spikes for livestock. Second, using table salt with iodine added — kills invertebrates within hours. Third, mixing too cold — salt dissolves slowly below 20°C and leaves crystals on the bucket bottom that release later as a slug of high salinity. Always mix at room temperature, 25-28°C ideal in Singapore.

Related Reading

• Sailfin Molly Brackish Care Guide
• Orange Chromide Cichlid Brackish Care Guide
• Biotope Mangrove Estuary Paludarium Design Guide
• Brackish Shrimp Amano Options Guide
• Cryptocoryne Ciliata Brackish Care Guide