Brackish Aquarium Setup
Most aquarists will easily recognize the standard tank types: Freshwater, Fish-Only Marine, Fish-Only-With-Live Rock, and Reef Aquarium. There's another type of aquarium, however, that is a bit more rare, though is gaining in popularity. This is the brackish aquarium setup.
Brackish water is somewhere between "pure" freshwater and ocean water in salinity. In nature, brackish water is created by swamps and marshes that are more or less at sea level and on a coast, estuaries, and, rarely, landlocked bodies of water that happen to have salt in them.
Why Brackish?Brackish aquariums have both advantages and disadvantages when compared to a freshwater aquarium setup or marine setup. They are a little more difficult to keep up than a standard freshwater aquarium, but are quite a bit easier to keep than a standard marine aquarium. There is a pretty limited array of fish that can live in a brackish aquarium, but the ones that will survive are often fairly hardy, there are several very peaceful species, and there are a few odd species that prefer brackish water. Brackish species also have the advantage of being very adaptable as far as water parameters go. As long as waste products (ammonia, nitrite, and nitrate) are all kept at safe levels, the fish will do fine if there is a change in pH, hardness, or salinity, as long as the change happens gradually. Because these fish naturally live in an environment where water parameters may change from square foot to square foot, a super-stable environment is not quite as important.
Brackish Tank SetupSetting a brackish aquarium up can easily follow the standard freshwater setup. The one thing to be sure about is that you are not buying any equipment that will be susceptible to damage from salt. It's best to buy aquarium filters that are for either saltwater or freshwater aquariums, and the same with any other equipment / chemicals you get.
The first place where the setup diverges from a standard freshwater tank is, of course, filling the tank. A typical brackish tank has a specific gravity between 1.005 and 1.010, but can range anywhere between true freshwater and true saltwater (up to 1.020). Most fish will have a preferred specific gravity, but as I mentioned above, most brackish fish are pretty adaptable in regards to this. It's best to do research into the fish you plan on keeping, making sure that all of the fish have compatible specific gravity requirements.
Once you've determined the specific gravity you're aiming for, use marine aquarium salt (not therapeutic stuff that is sold for freshwater fish), following the instructions that came with it. You'll need a hydrometer (or if you want to make things really easy and have some money to blow, a refractometer) to measure the specific gravity. Pay attention to the amount of salt you use. Because you don't have to maintain a very exact salinity in a brackish aquarium, you won't have to check the specific gravity of every water change, but you want to have an idea of how much salt per bucket of water you're going to add.
Be sure to mix the salt water in the bucket. Most salt water aquarists put a powerhead in the bucket, but with the smaller amount of salt used in brackish water, I've found that simply stirring the water in the bucket works.
Once you've filled the tank, you can follow any of the normal procedures to cycle the aquarium. The one caveat to this is that I'm not sure if one should use the marine or the freshwater version if cycling with Bio-Spira. I can make guesses, but because these are all pretty much uneducated, I won't even write them here.
What to Stock the Aquarium With?As far as choosing animals to put in your tank, as I said, the most important thing to do will be research. A few common brackish fish are guppies, mollies (these fish, common in freshwater tanks, actually prefer brackish water. Some species, such as the black mollies, do best in water that is very close to full saltwater), gobies (these fish run the gamut from full freshwater to full saltwater), and the dwarf puffer. These are only a few examples of brackish fish. Research can find even more (and even more odd, such as the archerfish) brackish fish.
In addition to fish, there are several invertebrates that live in brackish water. True Amano shrimp live in salt marshes, and are actually incapable of breeding without some salt in the water. Some fiddler crabs (including the ones usually sold in pet stores) live in brackish water. In addition, there are other shrimp, crabs, and snails that will do well.
Another thing to consider is whether or not you will want plants in your aquarium. As with animals, there are only certain plants that can survive brackish water. I have had great success with Java moss and Java fern attached to driftwood, and anacharis and milfoil have both done well floating in the tank and anchored in the gravel bed. A single bunch of micro chain sword has survived being in the tank, but has not grown yet.
UpkeepOnce your aquarium is set up, it is probably best to allow the specific gravity to vary periodically. Most parasites and bacterium have a more narrow range they can survive in than your fish, which means these changes help keep your fish healthy. Such changes will help mimic the naturally changing salinity that most brackish animals live in. The only time that this would not be good is with fish/critters that require a narrow range of specific gravity (I cannot find reference to such a creature, but there is enough variety in the world that I'm sure at least one exists).
Most of the problems that you will encounter in a brackish tank are similar to those that would be encountered in a freshwater tank, and can be addressed in a similar manner. If massive changes in water are needed for any reason, it is a good idea to match the salinity of the new water to that of the old water. While the fish are adaptable, there is a limit to the amount ot change they can handle in a short time.
This guide is not meant to be comprehensive in regards to keeping a brackish tank. Rather, it's meant to help you take the first steps if you want to. Once you get such an aquarium going, it becomes easy to incorporate the little bit of extra care it requires into your schedule, and the different creatures you can keep that couldn't thrive in a freshwater tank will provide endless opportunities to learn about and appreciate them.
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