What they said. you'll need several. I've got an Ehein 2213 with custom spraybar (thanks to Jake/somewhatshocked's journal) along with a dual sponge filter on one end and airstone at the other end. I put plants at the ends, in the middle, basically everywhere I can to see if there are deadspots anywhere. Moss is a good one to have around because it will sway easily with the current.
I also have a 2213 with a 30" custom spray bar lol. Also a danner mag 350 fractioning impeller on a closed loop with 6ft of head I was using for co2 but now just run. The danner is split to locline tgat goes to each corner to make up fir the shirt soraybar. Also have a sponge filter at one end and a marina breeder box at the other lol. Kinda over kill but it seems to be working.
I love the dimensions of this tank, it is one of the few tanks I kept running during my recent interstate move, but it is a pain to light and filter. If you are not going to go with anything but sponge filters I might consider adding a Koralia nano for some flow (they are pretty reasonably priced if you shop around).
If you can, iwould say go with a canister and spray bar. 3-4 sponges would look ugly imo. Youd be taking away a lot of the aesthetics of the tank. I running an eheim 2213 and the flow isnt too bad. I honestly think i could throw shrimp in there. Im gonna throw in ammanos for sure or maybe even some neos.
For a special tank like 12 gallon long don't use any hang on or sponge filter. Spend a little to get a small canister and glass intake & output. You'll appreciate it more when the tank look clean without anything obstructing it.