Ok I have been looking at canister filters for my tank. I currently have a freshwater filter for a 55 gallon tank. I am now saltwater and was about to purchase a saltwater filter besides a skimmer. The canister filters I have been looking at are basically Water in > Filter > Carbon Filter > Subtrate ----- then water out < Filter < Tank.

How come I can just add subtrate to my existing filter to make it work? Its the same concept except for the nitrate suplement.