Do Not use any kind of sponge, it will only hold nitrates. Unless of course you plan on rinsing it constantly.
When I first started in salt water I tried to use hang on filters, including the Brutes of the hang on "Supreme Super Aqua Kings". Even they didn't last long with the salt water, and these were filters that I had always used for my fresh water tanks. I use to raise cichlids namely Oscars. Therefore I would not recommend a hang on filter, but that is just my opinion.
On my old 75 fowlr, now a 120, I used the same skimmer I am now using on my 155 bow front (a mixed reef)- Turboflotor Multi 1000 with an Oceanrunner PH2400 pump. They run around $200, very easy to use. Virtually plug & play, and very versatile. You can hang it on the back of your tank, sump, or place it in the sump. You can pick them up for about $100 used.
If you'd like to see a pic of my turbo in action there may be one in my gallery.
Click here: Saltwater Marine Aquarium Water Quality & Filtration: Turboflotor Protein Skimmers
On my 120 fowlr I am running a
Red Sea Berlin also with an Oceanrunner PH2400 pump.
You can use air pumps in a salt water tank, but again that has it's own draw back - lots and lots of salt creep. I much prefer the power heads. And yes, they are used to move the water around.