Good tank theme? for 20 gallon tank?

freshwater white sand, black rock cave, as a center piece. 2 black moors, 1 rukin, dark-green-prickly fake plants. now i want sand just cause i hope to turn this tank into a brackish water tank soon. and sand seems very nice for that. my filter i am not worried about mixing the sand up and making it cloudy. and ya i know the tank is a tad small.