if it works flow rate would be pretty slow and I don't think it would be an effective filtration method you can buy cheap aquarium pumps on eBay that would be much more effective.
thanks brendan been using a couple hobs to get started but want a canister or a sump once i get both tanks going
replace your air pump with a aquarium pump.
just get a sump, the time and money you spend on that will be better suited doing other things, yo can always do an above tank sump so you dont need a real overflow.
Thanks jeff was leaning twards a overhead before but did not want to do it if it did not get recomendations
Lower the water level in the tank so that you can measure the amount of water flowing out. This will be the rate at which your filter works.
I stopped using air pumps last year because I felt water pumps would filter more water. They do, in fact, move more water but after cleaning my sponge filters I noticed that the sponges connected to air pumps were dirtier than the ones connected to water pumps. The slower rate actually aided filtration. So I switched back to air pumps.
I think you have a pretty good idea and the air pumps use much less electricity. As Peter suggests, Uaru Joey has some great DIY videos.