ok so i have another crazy tank idea, that i probably wont ever get to do, you take a 15 or 20 gallon and put some marine mud(the stuff for refugiums) and put about 6 inches of freshwater, so it turns to brackish, and make like a divider so about half the tank is just water and mud like a shallow fishtank,and the other side of the divider is solid mud. you take some root driftwood and cut it so it is like a bridge inbetween the 2 sides. on the water side you add some mudskippers and on the mud side you add some fiddler crabs. i will draw a pix and post it if i can.