Recently I set up an aquarium for my apartment. It’s a 25 gallon filled with a slightly exotic combination of salt and fresh water called “brackish” water. Anything with a salt content between 0.5% and 30% is considered brackish, and is found naturally in estuaries as well as some swamps, seas, and lakes. Many of the aquarium fish sold for brackish tanks come from mangrove swaps, an interesting tidal habitat where the roots of the mangrove trees protrude throughout the water.
I chose brackish instead of any other because you can find some of the most interesting fish in this type of habitat. My favorite, the green spotted puffer fish (Tetraodon nigroviridis), is very lively and playful. They’re fun to watch because of their brilliant green color and their endless antics—mine does loop-d-loops and is curious about anything new in the tank. They seem to be pretty much always hungry though and will eat anything in a shell, so keeping snails, crabs, or shrimps in the tank is a hopeless venture.
Another fish with a big personality is the green spotted scat (Scatphagus argus). Also green with spots and also endlessly hungry, these fish will go crazy for most anything you put in the tank. Their scientific name is Scatphagus, meaning "eater of feces," so it's no surprise that they'll go for about anything edible in the tanks. I like to stick a piece of cucumber slice on a post, so that it half floats in the water. The scat will feed off of it, looking like a boxer with a punching bag, swooping in and out as it bites at the cucumber.
Although I don’t yet have either, monos (Monodactylus sebae) and archer fish (Toxotes jaculatrix) are also brackish tank favorites. Both archers and monos come from mangrove swamps. In nature, archer fish spit down their prey (insects) from overhanging leaves. Monos are flat, sliver, and diamond shaped. Although tempting, they would get much to large for my 25 gallon tank.
Many hardier fish will also acclimate to brackish water. I currently have a pleco which are normally fresh water fish. He is quite the ready cleaner and will suck the algae off of the glass and fake plants.
Puffer picture: http://amize.jp/kuluri/index_en.html