Is it possible to make my freshwater tank into a saltwater tank

I want to make my freshwater tank into a salt water tank. It is only 20 gallons, but I want to get like clown fish and fish like that. I always take very good care of my tank, and fish. I have some freshwater fish, but I will wait till' they die off. (I know that sounds mean, but they will die eventually) I REALLY want this to happen. Would I need to buy a new tank, and filter? I want to use the one I have now.

It would be possible to convert your tank to saltwater, although a 20 gallon is a little small. You would be able to keep a pair of clownfish if these were ocellaris clowns (the "Nemo" type, as they only get around 2-3 inches), although other species that grow to 5-6 inches would be too large. Generally, the larger you go in saltwater the better, since the fish are less tolerant of changes to their environment, poor water quality, and overcrowding. The fish tend to be more aggressive/territorial, and most grow to a larger size than typical freshwater community fish, so this generally means much lighter stocking rates for saltwater. If your tank is a 20 regular/tall, that would be okay for the clowns. If it's a 20 "long" you might be able to keep another fish as well. I would recommend something very different than a clown for a third fish because their territorial/aggressive nature doesn't allow them to mix with fish with similar colors or body shapes, or members of the same family. This applys to most marine species, so for most groups, this means only one fish per type (tangs, triggers, wrasses, etc.) in the tank. There are a few species that can be kept as mated pairs (clownfish, cardinalfish), and even fewer that can be kept as a school.