mollies are a universal cleaner in my aquatic gardens. they breed like crazy in my brackish tank, and the fry feed my eel the other fish, and my mantis shrimp. a lil acclimation and there good to go from FW to BW and SW, and i use theme to clean algae everywhere.
after my nano reef cycled, i popped 3 1/2" fry in the tank and all the algae was gone in 72hrs.
guppies can also live in BW.
id go for a low SG (1.008-1.0012) get some java fern and bannaplants, BBG's (2-4) and some fancy guppies or mollies.