7 Brackish Water Aquarium Basics You Should Know 5

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If you’ve ever purchased a Dragon Goby, Colombian Shark Catfish, or Fiddler Crab for your freshwater tank and found that they never thrived, it’s because these (and many other “freshwater” fish and invertebrates) are actually brackish!

A brackish set up can be very interesting in their own right and can be a good stepping stone towards starting Marine tanks.

1. What Are Brackish Tanks?

A “brackish” system is identified by water that has a specific gravity between that of freshwater (a specific gravity of 1.000) and full saltwater (around 1.021-1.025).

brackishtank1(Image Source)

Specific Gravity is a measure of density of aquarium water relative to that of pure water and in brackish tanks is modified through the addition of Marine Salt Mix.

A hydrometer can be used to measure Specific Gravity and is available inexpensively at most pet stores. More accurate tools like refractometers are available and are often used for Marine Tanks for advanced hobbyists.

However, hydrometers are “good enough” for brackish tanks as these fish are less finicky about specific gravity than the often delicate corals and cnidarians kept in reef tanks.

2. Aquarium Salt vs. Marine Salt

Aquarium Salt (Sodium Chloride – the same as kosher salt and cooking salt) is often sold for use as freshwater medication.

Although this raises the salinity of water, it does not provide the same benefits of increase General Hardness (dH), Carbonate Hardness (KH), or any of the other trace minerals that is found in brackish water.

Marine Salt mix (a composition of Sodium Chloride, Magnesium Bicarbonate, Calcium Bicarbonate, and other elements) is needed for the long term health of brackish fish and invertebrates.

This mimics the water found in their natural habitats, where water from the ocean mixes with fresh water from rivers and streams.

3. Modifying Specific Gravity

Raising and lowering of Specific Gravity is accomplished through water changes. When you do your weekly maintenance, you add in water with a specific gravity that will bring your tank to where you want it to be. It may take a little math, but it’s not too difficult.

Many fish aren’t fussy about how quickly changes are performed, but for the benefit of all (including filter bacteria), it’s best not to jump more than .002 specific gravity at a time.

Let’s give an example: say you have a 40 gallon freshwater tank and want to convert to brackish without removing the fish. If you replace 10 gallons of fresh water (25% of the tank) with brackish water at 1.008 specific gravity, this will bring the tank to a whole up to 1.002.

The next week, you can bring to 1.004 by replacing 10 gallons of tank water with 10 gallons at about 1.010, and so on.

4. Cycling a Tank

Tank Cycling is performed the exact same way as in Freshwater and Marine tanks; the addition of an ammonia source over a few weeks to establish the bacteria that convert Ammonia to Nitrites, and Nitrites to Nitrates.

The process is simplified more if you are converting a cycled freshwater tank to low end brackish (at/below 1.005) or have cycled filter media. Just take your desired tank and add enough Marine Salt mix to raise the Specific Gravity by .002 per week.

The freshwater bacteria can adjust to these conditions and within two or three weeks you will have the specific gravity where you’d like it and a cycled tank, all with fish still in the system!

For higher salinity tanks, you can either go the same route (if fish are in the system) or for tanks without livestock you can add the salt mix in to get to the desired specific gravity and cycle normally.

5. Adjusting Livestock


For current brackish-tolerant livestock, the same increase in Specific Gravity of .002 per week will suffice for almost all species of fish and inverts. Certain fish are less demanding than others here;

  • the Target Fish (Terapon jarbua)
  • and the Molly (Poecilia sphenops, P. latipinna, and P. velifera)

are all famous for being able to thrive in Marine tanks straight from Freshwater conditions, but even the Volitans Lionfish (Pterois volitans) is found in estuaries noted for having .020 changes every 6 hours.

brackishtank2(Image Source)

For any new livestock purchased from freshwater (a common practice with “Freshwater” Morays and “Freshwater” Horseshoe Crabs, among others), they should be kept in a Quarantine tank and slowly acclimated to the main tank specific gravity.

If a fish purchased this way is having orientation or skin issues (owing from incorrect environment), adding additional Marine Salt to the tank should help correct this issue.

6. Marine Tools of the Trade

Although Brackish Tanks can be handled more or less like Freshwater tanks, at higher specific gravities you can utilize some of the Marine Tank standard practices.

Deep sand beds (also useful in Freshwater tanks but more associated with Marine Tanks) can finish the Nitrogen cycle by converting Nitrates to harmless Nitrogen gas (denitrification).


(Image Source)

At or above 1.010, protein skimmers can be utilized, albeit not as effectively as they are in Marine Tanks. Protein Skimmers are useful for removing dissolved organics from water and are more effective than Carbon filtration at this task.

A properly sized protein skimmer (rated for turnover about equal to tank volume per hour) can be very effective at keeping Nitrates and Phosphates low.

Around 1.015, Live Rock is usable (though it’s technically usable under this Specific Gravity, this level of salinity will keep the bacteria on the rock working more effectively). Live rock is used to assist in denitrification and can also provide grazing material for tank livestock.

7. Decorating

brackishtank4(image source)

Perhaps surprisingly, many plants available for freshwater tanks will thrive in low end brackish conditions (up to 1.003). Plants from the common genera

  • Anubias,
  • Cabomba,
  • Ceratophyllum (Hornwort),
  • Elodea (Anacharis),
  • Cryptocoryne,
  • Hygrophila (Wisteria),
  • and Vallisneria

will do well in these conditions, and Java Ferns will tolerate up to 1.005.

There are few available plants which tolerate more than this (including Seagrasses and Mangroves) but these can be very high maintenance and plastic plants may be better choices for most applications.

There are several options for Brackish Tanks that can’t support plants. Rocks are more common than Driftwood in these environments usually (though the water should be sufficiently alkaline to prevent a drop in pH from tannins) and sticking with a few large rocks of the same type is more natural-looking than several smaller rocks of different origins.

Some rocks, such as Limestone, have the added benefit of raising the hardness of the water.

brackishtank5(Image Source)

Shells can be useful; in addition to naturally being found in estuaries, they also keep the water hardness high and can provide places for smaller fish to hide. A common biotope is taking several oyster shells and using Cyanoacrylate (“crazy glue”, which is aquarium safe) to adhere them to driftwood or rocks to form oyster reefs.

The quality of aquarium ornaments has risen in recent years, too. In fact, fake Mangrove Roots are largely better than live Mangroves for all but the biggest tanks and most experienced aquarists. The fake roots even have better longevity than the driftwood shaped as such, as they do not dissolve over time and do not release tannins in the water.

Any substrate adequate for freshwater use is also an option for brackish tanks. Soft silica sand is best for low end conditions and burrowing fish (like the Spiny Eels of the family Mastacembelidae), though other options (such as Aragonite or Crushed Coral mixed in with sand) can be used with less delicate fish to keep the hardness of the water high.

Final Words On Brackish Tanks

The Brackish Aquarium is surprisingly easy to get started. Though there are some tricks depending on the set-up, for the intermediate aquarist with Freshwater or Marine Tank experience, there should be no significant problems moving to Brackish.

About Joel Haan

Joel Haan has been keeping fish off and on for over a decade, though his interest in Aquatic Science has been much more recent. His current focus is on Brackish Aquariums and he operates a blog on tumblr dedicated to them (thebrackishtank.tumblr.com). He currently keeps four fish tanks comprising various community fish species.

5 thoughts on “7 Brackish Water Aquarium Basics You Should Know

  1. Reply Don Bailey Jan 9,2016 3:09 pm

    Hi everyone!
    I’m brand new here and this is my first post. I am looking to start a brackish aquarium with all of the usual suspects {archers, scats, monos (both argenteus and sebae). a green puffer, bumblebee gobies, 1 pair black sailfin mollies, and some kind of cat(s)}. I am going for a 65 gal. and my questions are these: 1) I keep reading that (especially for scats and monos) the preferred tank size is much larger to accommodate their growth. I was under the impression that these (or any) fish will fail to reach full adult size given the constraints of a smaller tank. I have kept all of these spp. in a 55 gal. for years w/ no problem, so has the conventional thinking changed that much in the last 20 years? 2) I am also looking for a good cat/biologic algae-detritus control device . I seem to recall that I had a nice pleco that tolerated salt on the lower end of the brackish scale. Any thoughts? This looks like a great place–can’t wait to hear back! Regards, Don Bailey
    P.S.The tank pictured at the beginning of this article is pretty much exactly what I’m going for, with the addition of a few scats.

  2. Reply Mark C Feb 16,2016 10:14 pm

    Don, like you I have successfully kept monos and scats in a 55 gallon brackish tank for years with no problems. I have also read that I “shouldn’t” be able to do this. (shoot, back when I was young I had a 20 gallon brackish with monos, a figure 8 puffer, and bumblebee gobies). If experienced aquarists like us listened to the “conventional wisdom” about minimum tank size, etc, we wouldn’t have the awesome nanoreef aspect of the hobby like we do now.

    PS – can you really keep bumblebee gobies with monos and scats? What’s your secret? I have not had consistent success with bumblebees, seems they are really low-brackish fish while monos and scats are high brackish fish, and I also have trouble getting bumblebees to feed.

  3. Reply Cheryl Mar 31,2016 4:33 pm

    Thank you. This is the exact article I was looking for without knowing it. 6 months ago a coworker moved and gave me a 40 gallon tank with all the bells and whistles. It’s been running smoothly but I’m bored with it. I wondered if I could turn it into a brackish tank and your article answered all my questions. It will be fun to have a new aquarium to fiddle with now. I always wanted a little puffer fish and now I can get one! The only fish I have are black mollies, it seems like they may survive the transition.

  4. Reply Robert May 8,2016 5:33 am

    How do I turn my brackish fish back into freshwater I know I have to do it slowly but how slow is slow and how fast is too fast please let me know

  5. Reply Stephen Double Mar 26,2018 5:46 am

    I’m only using 1 gallon of water the rest of the tank is sand for my red claw crab tank.When I do water changes — how would it stay cycled with this amount of water?

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