What Is Brackish Water?

Brackish water has a salinity between freshwater and full seawater, the kind of mix found in estuaries and mangroves where rivers meet the ocean, made by adding marine salt to freshwater to reach a specific gravity usually between 1.005 and 1.015. In plain terms, it is a middle ground that is saltier than your tap but far weaker than a reef tank.

Brackish is a genuine third category of aquarium alongside freshwater and saltwater, with its own species and its own care routine. It is often overlooked, but for the right fish it is the only way to keep them thriving long term.

Where brackish water sits on the salinity scale

Salinity is most easily tracked by specific gravity, a measure of how dense the water is compared to pure water. Freshwater sits right around 1.000, full reef saltwater runs 1.020 to 1.026, and brackish fills the gap.

Because brackish spans a range, your exact target depends on the species you keep. Some fish, such as the green spotted puffer, prefer low brackish as juveniles and shift toward higher salinity as adults, so plan to adjust over the fish's life.

Classic brackish species

These fish evolved in estuaries and generally do best with at least some salt in the water. A few favorites for brackish tanks:

• Mollies, hardy livebearers that often color up and stay healthier with a bit of salt.
• Figure-eight and green spotted puffers, charismatic but messy fish that need salt and large tanks.
• Bumblebee gobies, tiny striped fish well suited to low-end brackish nano tanks.
• Archerfish, monos, and scats, larger active fish for big brackish setups.

Always research each species before mixing them, since salinity preferences and adult sizes vary. Puffers in particular are best kept alone or in carefully chosen company because of their nipping and high bioload.

How to mix brackish water

Making brackish water is the same process as mixing saltwater, just to a lower target. The most important rule is to use a marine salt mix, not aquarium tonic salt or table salt. Marine salt supplies the minerals and buffering that keep brackish water hard and alkaline, which is exactly what these estuary fish prefer.

1. Dechlorinate your freshwater, or start from RODI water for full control.
2. Add marine salt gradually and stir until fully dissolved.
3. Measure with a hydrometer and adjust toward your target specific gravity.
4. Match the same salinity for every water change so the tank stays stable.

To work out exactly how much salt your volume needs to hit a target specific gravity, use our salt mix calculator. If you ever need to translate between specific gravity, ppt, and other units, our aquarium unit converter handles the math.

Tools for Mixing Brackish Water

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Instant Ocean Instant Ocean Sea Salt (50 gal mix)

$22.37 on Amazon

Marine salt mix to build brackish water to your target specific gravity.

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Coralife Deep Six Hydrometer

$13.99 on Amazon

Swing-arm hydrometer to read salinity and specific gravity.

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Keeping brackish water stable

Day to day, a brackish tank runs much like a freshwater one with one extra habit: matching salinity at every water change. Mix replacement water to the same specific gravity as the tank so the level never swings. Keep a reliable hydrometer on hand, top off evaporation with fresh dechlorinated water (since salt does not evaporate), and confirm your readings before adding water.

Brackish fish are generally hardy, and the salt itself discourages some parasites, which makes these tanks rewarding once you have the routine down. If you are still deciding between water types, our guide comparing freshwater vs saltwater aquariums puts brackish in context.