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