Anything smaller then a 10 gallon is a sad life for any frog, no matter how small.

A small tree frog still needs space to climb/jump and a 5.5 gallon simply won't give him/her enough room to strech the muscles.

The only frog I'd ever put in such a small tank would be a mini africian clawed water frog. They won't grow more then two inches (be sure you get the mini ones, the other kind get quite large), move about as much as toads and will eat fish food, dried crickets, little guppies and even small worms. A small fish tank pump to keep the water moving will keep the tank clean and going for quite a while, requiring little cleaning.

I had two during college in a similiar tank and they did quite well for the 4 years in my dorm. Easy maitance and they actually are quite a cute little frog if you get the spotted gray ones (the Albino white ones scare me!, hah). Nothing more fun then watching them chase tiny guppies and gobble them down.

http://t1.gstatic.com/images?q=tbn:A...w_xKw5PjrtdvbQ
Nice face shot of one.

Some larger ones eating crickets, earthworms and krill, but you get the general idea how they move and feed.
YouTube - AFRICAN CLAWED FROGS EATING III