David has made some good points above. But I'll add one thing about brackish water aquaria -- to some degree, the direction the tank will go depends a lot on how good your local aquarium shops are. Some shops (like Wildwoods) regularly get in oddball brackish water fish every month, so it's quite easy to get some really cool stuff. Other shops only sell brackish water fish rarely, usually accidentally.
Now, the big plus with brackish water aquaria is the chance to mix "ordinary" fish with oddballs. Because brackish water communities are made up of most marine fishes able to tolerate reduced salinities, the diversity is very different to freshwater habitats. Instead of barbs, characins, catfish, and cichlids being dominant, you see examples of otherwise marine groups, things like morays, puffers, flatfish, gobies, sleepers, pipefish, and halfbeaks, plus various perciform and scorpaeniform groups. You also get some examples of things traditionally thought of as freshwater fish but equally at home in brackish (and sometimes marine) waters such as killifish and livebearers.
There are two ways to take a brackish water tank really. One way is the low salinity brackish water tank at SG 1.003-1.005. This tank can have lots of plants and bogwood, and apart from the small amount of salt (10-20% normal marine salinity) looks and works like a freshwater tank. This sort of aquarium can include livebearers, halfbeaks, glassfish, figure-8 puffers, certain cichlids (e.g., orange chromides), certain catfish (e.g., some of the banjo and Mystus spp. catfish, plus a few plecs and the Hoplo catfish), bumblebee and knight gobies, spiny eels, and even a few loaches and barbs, notably horseface loaches, ticto barbs (Puntius ticto), and blackspot barbs (Puntius vittatus). Most of these fishes will get along, though some (like knight gobies and Mystus spp.) are predatory and will view small gobies and livebearers as food!
The other way to run a brackish tank is at a fairly high salinity, say SG 1.010-1.012 (around 50% normal seawater). This tank would be based around those species that migrate in and out the sea, and these fishes tend to be large and rather robust. Monos, scats, and archers are the classic species for these tanks, but you could also keep green spotted puffers, a few catfish (such as shark catfish and Mystus gulio), moray eels, certain cichlids (Sarotherodon melanotheron is my favourite brackish/marine cichlid), large sleeper gobies, waspfish, and violet gobies. With care, there are also some species sold as tropical marines that adapt well to these conditions, including some of the snappers, puffers, batfish, and a few inverts, notably blue-legged hermit crabs. Plants are generally not an option here because of the salinity level, and many brackish water species simply eat plants anyway so it isn't worth the bother. But you can make up for the lack of plants by using large rocks, artifical mangrove roots, seashells, and so on.
A 50 gallon tank gives you a lovely canvas to work with. I'd personally recommend opting for the small-fish, low-salinity option. Getting those species is generally easy, and you can use things like mollies or guppies to mature the filter without any problems at all. It would be easy to fill the tank with Java fern and other hardy plants to build up a nice tangle of vegetation, and then you could be reasonably safe keeping figure-8 pufferfish alongside other species knowing than the size of the tank plus all the hiding places would be keeping everyone happy.
Cheers, Neale
