What level of salinity will the brackish setup be? What fish do you want? There are some marine snails that will tolerate high brackish conditions, but finding plants to suit will be tough. There are a number of fish--mollies being first on the list--but a lot will depend on what other fish you want in there. I think scats will also eat algaes--but get to be substantial fish, so won't work if this is a small tank.