When it comes to setting up a brackish tank, there are several choices. It all depends on the type of fish you want.
1) If you want larger fish, and therefore a larger tank, your main choices are monos (monodactylus argenteus or sebae or others), scats (scatophagus sp.). Archers (Toxotes sp.), Dantoids (Datnioides microlepis), silver sharks (Arius seemanni), and chromides (Etroplus sp.) This type of tank should be rather large, and have plenty of driftwood. There are several species of plant that will do okay in brackish water, but you either need to introduce the plants very early so they can get settled, give them pristine conditions, or constantly have plenty of plant matter in food, bebecausecats and monos are voracious plant consumers. You will also need a good filtration system bebeaushese large fish large "outputs". Food for the tank should be spirulina flake or pellets, freeze-dried frill or other food, sinking wafers for the catfish, occasional meaty food, and crickets for archers.
Archers-kept either singly or in a group, not pair or three.
Monos and Scats-do best in groups, though can be kept in smaller amounts.
Dantoids-kept singly, or several in large tank.
Shark Catfish-group of three or more.
Chromide-best in group
2)The smaller brackish fish just as plentiful as the large. Bumblebee and knight gobies, as well as other gobies, mollies, halfbeaks, celebs rainbowfish, glassfish, flagfish, and moquito fish are some of the smaller fish for smaller tanks. The gobies to well in sand substrate. The moquito fish, mollies, and halfbeaks are all livebearers, and the flagfish are a type of killifish. All these fish stay relativly small. One piece of advice is to stay away from PAINTED glassfish. These are any with the flourescent coloring. Most will end up losing the color and dying due to the color injection process. These fish are mainly carnivorous, and an occasional serving of baby brine will be greatfully accepted, especialy by those with the smaller mouths (i.e. gobies) that cannot eat the adults. Some of the gobies will not or have to be trained to eat prepared food, so froven/live is your best bet.
3) The low water tank provides a way to see the natuaral habits of some brackish fish. Mudskippers are well known for coming out of the water, and do well with a tank that is partialy land with the addition of rocks sticking above the water for them to climb on. A great plants for this setup are mangroves, a plant that keeps its roots partialy submerges in water, and does well in brackish or marine. The reason it is good for this style tank is that is tank room to grow upwards without burning its leaves on your light fixture. Other tankmates for this tank would be fiddler crabs, but make sure that there are slippery rock area for mudskippers to go that the crabs can't. Other good fish for low water (but big) tanks are the four-eyed fish and archers. Archers naturally spit water at insects to knock them in the water, and overhanging mangroves give them the perfect chance. four-eyed fish do not actually have four eyes, but each eyes has an hourglass-like shape giving them the ability to see above and below the water. These do well is a tank that is lowered so they are not floating right next to the light. For filtration a canister filter is best, though an internal filter could clean a small mudskipper tank.
4) Last, there are some wonderful brackishfish that unfortunatly, do not do well wityh others. In a speciies tank, however, you can keep several of these togeher happily. The most common are the various species of puffers, that in a tank with other will usually inflict wounds on other non-puffer tankmates. Another is the brackish pipefish that, like there marine counterparts, need to be in a tank with no competition for food.
For the basic care of any brackishwater tank, it is eccential to maintain a high pH and hardness level. This can be accomplished two ways. One is to use chemical liquid, powder, or tablets that raise the pH and hardness. The easiest way is to used crushed coral mixed in your gravel, or use coral sand. Using only coral substrate is usually not a good idea because in can increase the pH too mush, which should be maintained at 7.5-8.0. The next important step is salinity. This is remidied by using a marine salt at about half or less stregth as saltwater. the salinity should be around 1.003-1.10. However, as fish like monos and scats grow up, it is best to acclimate them to pure saltwater. Other braskish fish can usually be acclimated to fresh or marine too.
My last bits of advice are:
Do not buy monos, scats, archers, dantoids, and shark catfish based on how big they are, consider how big they will get.
Sure some brackish fish will live in pure freshwater, but few will ever thrive.
Do not throw in freshwater fish into brackish water for the same reason.
DO NOT impulse buy the exotic brackish fis (i.e. waspfish, bracksih moray, toadfish) often they get too big, have specific conditions needed, or bad attitudes.
Do not swear by my advice
There are always exeptions. Sometimes a puffer will live with a goby, an archer will do good as a pair, or a scat wont eat your plants. These are guidlines of what normally occurs
And I think that is it. I urge other people to come and correct me or add other advice!
P.S. what type of tank are you planning?
P.P.S. What? I am not long winded