If the tanks 3' x 3' x 1.2', then it's an ideal size for quite a few brackish water fish. The obvious choice are mudskippers, that need a tank with some water and some land. With a square aquarium this size, it should be quite easy to do (build up a mound on one side) and still leave space for some swimming room.
A lot of people recommend keeping crabs and small fish with mudskippers, but I wouldn't. On the other hand, the common mudskipper
Periophthalmus barbarus is very hardy and easy to keep. Half-strength seawater, hard alkaline water, and plenty of warmth and humidity, is all you need. Definitely visit Richard Mleczko's
web site; he knows more about mudskippers in captivity than anyone else on the web.
Another option would be a tank for flatfish and gobies. Flatfish are very cool and not difficult to keep. Many do well in brackish water, certainly the commonly traded species. To see something of the variety, check out
this page (in Japanese, but the pictures still work!).
Gobies, like
Awaous spp., and 'violet gobies', will spend a lot of time shoveling sand about. In a big, broad tank they could have a lot of fun, and you'd have an interesting tank to watch from above as well as from the sides.
If you can find the brackish water banjo catfish (not the small freshwater ones), these would work in this sort of tank too. Check out the
Brackish FAQ for stuff on these fishes.
Cheers,
Neale
