Those fire eels are huge, was tempted myself to get another eel from there. Remember eels need bloodworms and shrimp etc, they dont eat flake, although you probably already knew that anyway just thought I'd mention.
Edit: and eels dont like ammonia/nitrite, so yeah filter first dude, oscars will be fine now, but in a few months when they start chewing on the corydoras you'll certainly need to change something. Wanna work out what kind of tank you want.