Brackish Aquariums at a Glance
Brackish tanks offer a unique biotope opportunity — species found nowhere else combine brackish-specific behaviors with manageable maintenance.
Brackish Species Hardiness
Brackish Species Comparison
| Species | Max Size | Min Tank | SG Range | Temp °F | Notes |
|---|---|---|---|---|---|
| Molly (Sailfin) | 5" | 30 gal | 1.005-1.025 | 72-82 | Brackish improves health |
| Knight Goby | 4" | 20 gal | 1.003-1.010 | 72-82 | Bottom dweller, peaceful |
| Figure 8 Puffer | 3" | 15 gal | 1.005-1.012 | 74-82 | Semi-aggressive, needs snails |
| Green Scat | 12" | 100 gal | 1.005-1.025 | 76-84 | Grows large, needs group |
| Archerfish | 8" | 100 gal | 1.005-1.015 | 77-84 | Surface feeder, needs height |
| Bumblebee Goby | 1.5" | 10 gal | 1.003-1.010 | 74-82 | Nano, peaceful, shy |
Brackish Tank Setup Guide
| Mix saltwater with RO/DI or dechlorinated freshwater to target SG 1.005-1.010 | |
| Use marine salt mix (not aquarium salt) for proper mineral balance | |
| Add mangrove propagules for natural filtration and unique aquascaping | |
| Increase SG gradually (0.002 per week) when acclimating freshwater fish | |
| Provide hardscape and hiding spots — brackish fish appreciate structure | |
| Use 'aquarium salt' (sodium chloride only) — it lacks essential trace minerals | |
| Assume all freshwater plants work — many melt at brackish salinities | |
| Keep pufferfish without providing snails for beak maintenance | |
| Forget that scats and archerfish grow large — plan the tank accordingly |
Brackish is not a 'halfway house' for sick fish — it's a specific biotope with its own water chemistry, bacterial communities, and species requirements. Research before building.
Mollies (especially sailfin varieties) thrive in brackish water — they show better color, fewer diseases, and breed more readily at SG 1.005-1.015 than in freshwater.
Brackish water -- a mix of fresh and salt -- is one of the most underrepresented aquarium niches. It's home to some extraordinary fish: figure-eight puffers, archerfish, mudskippers, and dragon gobies. The challenge is that brackish conditions require more research than freshwater but less equipment than marine -- and most aquarium resources skip it entirely.
| Species | Adult Size | Target SG | Notes |
|---|---|---|---|
| Figure-eight puffer | 3 in | 1.005–1.008 | Solo only; needs snails for beak wear; 20–30 gal per fish |
| Archerfish | 8–10 in | 1.005–1.010 | School of 5+; 75+ gal; tight lid (jumpers) |
| Dragon goby | 20+ in | 1.005–1.010 | Blind filter-feeder; deep sand; often sold as freshwater fish |
| Green spotted puffer | 6 in | 1.010–1.018 | Adult needs marine-level salinity; more aggressive than F8 |
| Knight goby | 3 in | 1.005–1.010 | Peaceful; mix with scats/monos at similar salinity |
What Is Brackish Water?
Brackish water has salinity between freshwater (0 ppt specific gravity 1.000) and full marine (35 ppt, specific gravity 1.025-1.026). Most brackish aquariums target 1.005-1.015 specific gravity. The exact salinity depends on the species -- different species come from different salinity zones within estuaries, mangroves, and tidal rivers.
Salinity levels in nature: full freshwater rivers transition to mangrove creeks (low brackish: SG 1.002-1.006), then estuaries (mid: 1.006-1.012), then near-shore coastal areas (high: 1.012-1.020). Species often originate from a specific zone but tolerate a range.
Essential Equipment
Brackish aquariums need a hydrometer or refractometer to measure salinity. Hydrometers are inexpensive but less accurate; refractometers give more precise readings and are worth the investment. Check specific gravity weekly -- evaporation raises salinity as water leaves but salt stays behind. Top off with fresh water, not salt water, to maintain target salinity.
Substrate: fine sand replicates the mudflats and mangrove bottoms most brackish species come from. Crushed coral or aragonite sand buffers pH upward, which is appropriate (most brackish species prefer pH 7.5-8.5). Avoid acidic substrates like planted soil.
Filtration: Brackish fish tend to be messy eaters and produce significant waste. Canister filters or oversized HOB filters work well. Biological filtration is essential -- cycle the tank fully before adding fish. Salt does not deter the nitrogen cycle.
Species Profiles
Figure-eight puffer (Dichotomyctere ocellatus): One of the most popular brackish fish. 3 inches; intelligent, personable, and capable of recognizing their owners. Requires SG 1.005-1.008; pH 7.8-8.3; temperature 75-82 degrees F. Carnivores requiring live or frozen prey: snails (essential for beak wear), bloodworms, krill, clams. Should be kept alone or in a species tank -- they bite fins of any tankmate. Need 20-30 gallon minimum per fish due to territorial aggression.
Archerfish (Toxotes jaculatrix and T. chatareus): The fish famous for shooting water jets at insects above the water's surface. 8-10 inches; need a larger tank (75+ gallons) with space above the water. SG 1.005-1.010; surface area and a tight-fitting lid (they jump) are essential. Schooling -- keep 5+. Will eat any fish small enough to fit in their mouth; keep with similarly-sized fish only.
Dragon goby / violet goby (Gobioides broussonnetii): A fascinating eel-like goby reaching 20+ inches. Actually nearly blind and filter-feeds detritus from sandy substrate. Docile and peaceful. SG 1.005-1.010. Needs a long tank (48+ inches) with deep fine sand. Often mislabeled and sold at freshwater; does poorly at 0 salinity long-term.
Green spotted puffer (Dichotomyctere nigroviridis): Similar in care to figure-eight puffer but grows larger (6 inches) and requires higher salinity as an adult (SG 1.010-1.018). Juveniles tolerate lower salinity. More aggressive than figure-eights -- usually solo only.
Knight goby (Stigmatogobius sadanundio): 3 inches; peaceful and subtly attractive. SG 1.005-1.010; pH 7.5-8.2. Can be kept in small groups; gets along with scats, monos, and mollies at similar salinity.
Mono / Monodactylus argenteus: 8 inches; a schooling silver fish needing a large brackish to marine tank (100+ gallons for a school). SG 1.010-1.020. Often available as juveniles that look simple but grow large. Excellent for large species-focused brackish displays.
Compatible Plants and Decor
Most freshwater plants fail in brackish conditions above SG 1.004. The exceptions: Java fern, Anubias, and some Vallisneria tolerate low-end brackish. Mangrove propagules can be grown in brackish tanks with their roots submerged and leaves above water. Artificial plants work fine for species-only setups where realism isn't a priority.
Decor: driftwood (raises tannins, slightly acidic -- counterbalance with crushed coral substrate), ocean rock, ceramic structures. Rocks used for marine tanks work well.
Mixing Saltwater
Use marine salt mix (Instant Ocean, Reef Crystals) -- not aquarium salt or table salt. Marine salt mix provides the full spectrum of ions present in brackish/coastal water. Aquarium salt (sodium chloride) is an inadequate substitute for sustained brackish keeping.
To mix: add marine salt to fresh water and mix thoroughly until dissolved and temperature-matched before adding to the tank. Never add undissolved salt directly to a tank with fish -- osmotic shock risk.
Sources: Dawes J, 'Complete Encyclopedia of the Freshwater Aquarium' (Firefly Books); Seriously Fish species profiles -- Brackish; Evans DH & Claiborne JB, 'The Physiology of Fishes' (CRC Press); Practical Fishkeeping brackish species guides.