oscar friends

Question
hi
i have a large tank with 1 oscar ( 3 inch ) 2 tinfoil barbs (  6 inch ) and 2 plecos ( 6 inch and 12 inch ). and i wanted 1 more fish. i am worried about the compatability mainly with my oscar  and i hoped you could suggest something. a friend suggested koi as she keeps them  with  arowanas and cichlids  and told me they could adapt to warm waters is this true . i hope you can suggest a fish as i would love one more ( i do like  the look of gars  are they compatable)
so if you could say any fish that could possibly work and i could see which one i would be interested in
thanx  from james

Answer
Hi James,
This is going to be difficult because you already have your current fish established and they may be territorial (mainly talking about your Oscar) However, you do have an advantage because your fish are still young and they may be more accepting of a new tank-mate.

I wouldn't really suggest Koi, they would need such a massive tank and they of course do best in unheated tanks. Some fish you could try with your others are: Catfish such as the larger Synodontis species, Clown Loaches, Silver Dollars, Severums, Jade-eyed Cichlid, Chocolate cichlid, Banded Leporinus, Firemouth cichlids, Convicts, or you could try similar south american cichlids. One thing you might want to keep in mind is to try to get tankmates that are ideally already a little larger than the oscar himself, this is because oscars often grow faster than most of their tankmates. And getting those fish that are already on to a head start can help avoid some of those potential problems.
You could always try a gar, but keep in mind with cichlids, you never know if a problem may pop up. It's very difficult to determine and you have to keep a close watch at all times to make sure everyone is still getting along.

I really hope this helps!
Best of luck!
Karen~