I agree with PoPoV
JD and Oscars are very messy fish, IMO they both are very active when younger but just kind of sit around when older. A GT would be a nice addition, Im a big GT fan, Severums are good as well as Blue JD's.
as for filtration goes this all depends on you and how well you keep up with your tank. there are good arguments on both side of under gravel filters as to whether they are good to have or not. if you put in under gravel filters dont just assume that food and poop wont collect down there. I myself am not a under gravel filter fan at all, but its up to you.
), However, I want to know what other members on this forum think about what I should do with the tank. 
I wish i could fit a 120g in my condo 
As far as Oscars and JD goes, remember they are very messy fish. So I would recomend LOTS of filtration. Also Substrate that is easy to clean, I'd wait till a more expeirienced member chimes in on substrate as i dont want to give you bad advise. Iv used bigger polished river pebbles befor, and it was the bomb for messy fish. Have you ever looked into Green Terrors? they are much more beutiful and intellagent than JD IMHO. They also build a better fish to owner relationship from what I have experienced. But as far as Oscars and JD go, be ready for a commitment cause they are like kids. Iv had both and they demand CONSTANT attention. Good luck, hope that helped a little........Oh ya, no plants! JD's and oscars will dig them up in no time and make a mess. If you must have plants try java moss or somthing that can attacth to rocks 
