Freshwater Tank.

arrowana's get big. what size tank are you looking at?

arowana get into the 3-4 foot range so way to big for your tank

I can't think of any eels that would work in such a small tank. Do you already have the aquarium? You need a filter, some water dechlorinater and probably a heater unless you want to keep coldwater fish.
Have you researched how to cycle an aquarium and set up a new tank?

DPs stay small but they are not good beginner's fish. They are a challenge to feed. www.dwarfpuffers.com is a good place to research them. Your tank would be large enough to house several if you are up to the challenge.
I would really recommend an easier to keep fish to start out with. Guppies, Tetras, or just a couple of goldfish, for example.

Before I buy any fish I do my research. I first figure out what the scientific name is (genius and species) and then google the name. If I find contradictory info or not much info I ask people about their experiences with that particular species. When I stock the tank I plan it out to where the tank won't be overstocked at the fishes maximum size. You have about a 25g tank so plan your future fish that wont out grow this tank, unless you plan to up grade.