Yikes! I don't know why I haven't been getting updates on this thread! Looks like it's been hopping over here - no pun intended ;)
Jennifer, as far as plants are concerned. . . a lot of this depends on the type of water and lighting that you have, but one of the easiest plants to keep alive without help is Hornwort. It's also a great nutrient hog, and will help keep your water clear of toxins and such. The problem with it is that it's messy! My frog loves it, though. There are tons of options out there for floating plants, though. If you want to post up some of the specifics on your tank, I can try to help a little bit more in this. Not only will the frog love the floating plants, you can 'plant' them in the substrate or weigh some down, and they'll really love that, too!
As far as filtration goes, yes - have some! Though it isn't 100% necessary, it will make it a lot easier to keep the water properly maintained without a hitch - and we can use all of the help we can get sometimes! Be careful about the intake on your filter, though. It's a good idea to cover it with nylon mesh or a sponge to keep little flippers from getting sucked into it. . . Like Gizmo said, I would leave him be until you get a bigger tank. Moving them around too much isn't good, either - and he's probably fine in the 2.5 all alone!
Food. . .Stick with something designed especially for the frogs. If you can't find the HTH, Topfin brand sells one called 'Frog, newt, and tadpole bites' that are fine for ADF. Most PetSmart stores carry them - they're easy to find. But if you're going to PetSmart, ask for the freezer section and get some frozen goodies, too! I wonder if you'd be better off trying the frozen bloodworms? This can vary, but MY frog absolutley will not eat pellets under any circumstances EVER! He was never fed pellets though - in the shop they fed him brine shrimp, so I'm not sure if he even 'gets' that the pellets are food.