So you know what freshwater fish are, and you know what Saltwater fish are. You probably have a tank or a few devoted to one or both types of these fish. If you have experience in either of these areas you have probably heard the term "brackish" thrown around. But just exactly what is brackish?
            

                Basically brackish water occurs when the sea mixes with fresh water...it occurs in estuaries, aquifers,salt marshes, and mangroves most commonly. Areas where freshwater mixes with ocean water. Brackish water is not as salty as seawater but it is not pure freshwater. These areas team with life in the form of saltwater fishes, freshwater fishes, and true brackish fish, with a number of invertebrates as well. A wide range of salinity occurs in brackish conditions ...it is every range in between pure saltwater and pure freshwater, and certain brackish fish have certain preferences for the range of salinity they like to be kept at. Therefore one must be careful to house together only brackish fish with a compatible range of salinity. That being said brackish fish are extremely versatile; many being able to adapt to pure fresh or saltwater...and they are very forgiving of salinity fluctuations. In fact fluctuations within their prefered salinity range is normal and healthy for them. For this reason brackish fish keeping is more forgiving then true saltwater in which rigidity of parameters is essential to success. Brackish fish keeping is a great bridge from freshwater to saltwater,or for the unexperianced aquarist who is interested in keeping a marine tank. And it is a great and very overlooked branch of the hobby in and of itself. 

                                                                                                                                                 One bit of confusion that many hobbyist have is over the types of salts that can be used to make brackish water. While some care guides or retailers may recommended the use of aquarium, kosher or rock salt. However these are inappropriate substitutes. A brackish fish may be able to live in water with aquarium salt but he will never be as healthy or comfortable as in true brackish water. There are many components of true saltwater and the only adequate substitute for it is a true Marine synthetic salt mix.

Some brackish fish include:

Mollies
Halfbeaks
Certain types of Goby
Monos
Scats
Archerfish
Colombian Shark
Certain types of Puffer (Figure Eight, Green Spotted, Ceylon)
"freshwater" Flounder
Orange Chromide Cichlids
Green Chromide Cichlids
Kribensis Cichlid
Glass Fish
Some types of Rainbowfish
Mudskippers
Four eyed fish
Peacock Eel
Florida Flagfish
Targetfish

Some brackish/brackish tolerant invertebrates include:

Amano Shrimp
Colombian Ramshorn snail
Red Clawed Crab
Fiddler Crab
Blue-legged Hermit Crab
Mangrove Horseshoe Crab

This is only a fraction of the fishes that may be kept in brackish aquaria. I promise you'll have a suprisingly broad selection if you are starting a brackish tank. Remember to only keep fish of compatible salinity ranges and temperaments together...research the type of fish you are interested in! I can't stress that enough! Some types of brackish fish like Monos and Scats grow quiet large so make sure your tank is of adequate size for the type of fish you intend to keep.

Supplies for starting a brackish setup:

1. Appropriately  sized tank with a secure lid...Fish Jump!
2. Appropriate sized and type of filter. A nice hang-on back or canister filter is probably ideal.
3. Submersible heater and thermometer
4. Water conditioner...Brackish fish don't like chlorine either!
5. Freshwater/Saltwater Master test kit...Choose one appropriate for fresh or salt water! You need tests for Ammonia, Nitrite, Nitrate, and PH. Tests for GH and KH are optional but reccomended.
6.Never-Seen-Soap bucket for Water changes
7.Never-Seen-Soap Bucket for mixing up brackish water
8.Appropriately sized Gravel Vacuum
10. Marine salt mix...NOT aquarium, Epsom, or table salt!!
11. Hydrometer or Refractometer...whichever you prefer (Hydrometer is cheaper)
12. appropriate substrate and decor (Varies by brackish fish being kept)
13. Salty Bio-Spira, pure ammonia for fishless cycling, or a hardy brackish fish (such as mollies) for "natural" cycling.
14. 5-20 gallon quarantine tank for when you want to add to your established brackish tank...recommended quarantine period 4-8 weeks. (Many brackish fish are wild caught and may carry parasites)
15. Food appropriate to the type of fish being kept (varies by species)


How to mix up Brackish water:

1-2 tablespoons of marine salt per gallon should give you a moderate salinity of 1.010-1.017 depending on the brand of salt mix and how long the package has been open. Measure Specific Gravity on your hydrometer and adjust to the preferred range by adding either salt of freshwater  until the appropriate level is obtained.

Another way to mix up brackish water is to mix up saltwater according to the salt manufacturers instructions (Usually 1/2 cup per gallon), then mix with freshwater to achieve the desired Specific Gravity.

with the appropriate amount of dechlorinated water.
measure and add salt mix
Mix it up
Let it sit at least 12 hours to ensure that the salt mix is well dissolved.

For more sensitive fish add a heater to the bucket to ensure that the temperature of the water matches the tanks exactly. In any case temperature should be within a few degrees of the tank.

Remember! Salt does not evaporate! When "topping off" to make up for evaporated water always add freshwater.

Occasionally you may need to read the specific gravity of your tank. Specific gravity is the density of the water caused by the concentration of salt. to use a standard swing-arm  Hygrometer:

Collect the water to be tested in a jar or bucket. Water should be of the temperature range reccomended by the hydrometers manufacturer.Water being tested must have no current.Fill the hydrometer with water and place on a level surface. The arm swing to will indicate the specific gravity of the water. Rinse hydrometer with warm tap water befor and after each use. (most hydrometers require the water to be within a certian temperature range to read correctly....make sure the temperature range on your hydrometer is compatible with the temperature in your tank)


Now you're ready to keep a brackish tank!!!