The gear, placement/install and initial running in of brackish water systems is somewhere in-between that of a "typical" freshwater and marine array... Actually, more like the latter than the former. As you will see, brackish tanks generally need to be larger, and have similar provisions for stability, as most marine set-ups call for.
Tanks: Size and Shape Matters
The majority of popular species of brackish water fishes kept: scats, Monos, archers... are fast moving, heavy eating/defecating and largish animals. They need room. What's more you'll likely want to add decor items, maybe even take a shot at a biotopic presentation with submerged roots, rockwork, plants... that require an aquarium of size, of a more "flat" (i.e. not show, tall) profile. More surface area is desirable so a "stockier" more wide tank is preferable, for gas exchange, room for your fishes to move about, and for your aquascaping expression.
Harkening back to the issue of stability/homeostasis, larger volumes are inherently more so. Thus even if keeping smallish partly marine gobies, puffers or such, a tank no less than twenty nine gallons is a good minimum size. For a "beginner" brackish water system my suggestion for size is the same minimum as for marines, forty gallons.
Though many brackish water areas are mangroves, openings to the sea, coastal rivers... these lie within the same bright tropics as reefs, most of the organisms do best in medium to low light conditions or at least in tanks with a shaded region/area. Yes, a good deal of partially marine livestock lives along the darkened edges of such environments.
As a rule of thumb, 2-3 watts per gallon of normal output fluorescent or about half of this for boosted fluorescent lighting types... or for very deep tanks (more than two feet of water), metal halides... is about right.
Unless you're utilizing types/species of plants that require more intense lighting (in turn providing shade below) keep illumination of your brackish system low intensity.
Also needs to be heavily focused on stability. It is imperative that biological filtration (to most folks simply ready conversion of ammonia to nitrite to nitrate) occur expediently, w/o cessation. This can be a tricky proposition with brackish systems with varying water composition, ionic make-up and density especially. The microbes that are biological filtrations mechanism can be dealt serious population and declines and metabolic checks with too much change/vacillation, too soon.
The easiest way around these potential difficulties (interruption of biological filtration) is manifold: First and foremost is to keep the overall system stable. Larger volumes, redundant mechanicals, timers on lights, regular maintenance, water changes with pre-made and stored brackish water of desired make-up are principal.
Secondary to this are touched on under "redundant" above. More than one source of biological, mechanical filtration are especially warranted. Ideally live sand beds and rock (yes, here for the same reasons as its use in marine systems) can be employed in a remoted sump/refugium for ease of maintenance and flexibility, rather or in addition to their use in your main/display system. Other media (ceramic, glass bead, fused glass... can be used as back-up in a water-circulated device/filter (canister, hang on back power filter...).
To re-emphasize, the important points, watch words of brackish filtration are stability through redundancy and over-sizing.
Like filtration, should be "beefed up" and complete... transporting wastes, uneaten foods out of the system, moving all the water in the system about at a vigorous clip.
Factoring in that brackish fishes are messy, big eaters, tend to be more on the larger, more active end of the spectrum, it's easy to understand the logic of using a calcareous gravel to fight/balance the reductive (acidic) influences of such systems. Crushed coral, aragonitic materials, natural gravels are suitably alkaline. If not using live plants (which you should), an inch of so of about 1/16" to 1/8" diameter grain size is about right functionally, though many possibilities of types, shapes, depths of substrates exist. For instance, the use of live plants may call for deeper, finer beds with soil mixed into a lower region, and areas with naturally hard, alkaline water likely have much less need for calcareous substrates.
Adding a good amount of rock, submerged wood, live and/or plastic plants and more a plus with your brackish water system. Almost all brackish organisms come from habitats with substantial cover. Do your homework here in investigating how the life you intend to keep makes its life in the wild.
As with the wider brush of freshwater tropicals, there is a range and specificity of brackish water qualities... different pH's, hardnesses, temperatures, salinities... The general rule is to make and keep your water hard, alkaline and to a degree salty. But it bears repeating that some life is more tolerant in its acceptance, preferences and rates of change in these and other variables of water quality. Actual wild- recorded values by species can be found (e.g. www.fishbase.org).
The type/source of salts is important. Unless you're barely salting your water (say a teaspoon per gallon) do not use "sodium chloride" salts (table salt, kosher salt, ice-cream salt)... as these have additives your livestock doesn't need (iodide, dextrose...) and lack buffers, other ions from other salts that make up natural seawater. Instead, either utilize a synthetic salt mix intended for marine aquarium keeping, or if you have a good, clean source, natural seawater, which you can mix with fresh.
A resounding theme of brackish aquarium keeping is stability. I cannot emphasize enough how important keeping water quality constant is nor the simplest means to achieve it: Careful set-up, adequate filtration, circulation, aeration and regular water changes executed with pre-made, stored make-up water.
Re the last, a dedicated container, lid, powerhead/submersible pump, heater... on their own power strip are ideal for this preparation/use. With a few to several days to match temperature and specific gravity there may be no other chemical use than the simple alkalinity booster that is baking soda (sodium bicarbonate).
As you will find, the rules and procedures for setting up, establishing a brackish system lie somewhere between those of generalized marine and freshwater protocols, though closer to marine in the need for diligence. The life that hails from these habitats enjoy larger, less changeable conditions than typical freshwater biotopes.
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