Just to be clear, there's three basic types of filtration in these tanks:
- Mechanical filtration - things like socks, filter floss and pads all physically trap particles in the water and need to be replace often.
- Biological filtration - where beneficial bacteria process dissolved waste products. Rocks and sand are the primary, bioballs and the like also can be used.
- Chemical filtration - includes media like Granular Activated Carbon (GAC) and Granular Ferric Oxide (GFO) that remove things by reacting with them.
Please know that using granular activated carbon as a chemical media in your filter is NOT the same thing as carbon dosing, which involves adding vinegar or vodka to your tank as a source of elemental carbon.
Likewise, water flow in the tank is a separate discussion and essential if you want to keep corals.
And just to add on, you really need to start with purified freshwater to make new saltwater and to top off evaporated freshwater. Most of us have a Reverse Osmosis with DeIonization (RODI) filter to make freshwater for our tanks. If you don't have an RODI filter yet, you can buy gallons of distilled water at any grocery store or pharmacy.
Good luck!