RO isnt a powder. RO is water. You either filter this with an RO unit plmbed into your exsisting water supply. Or you purchase RO water from your local shop per gallon.
Sand? Unless you want a bare bottom tank or even a rubble zone tank.
Heater?
Powerheads, Saltwarer thrives in high circulation so you need alot of flow, far more than you would consider safe really for a tank.