Agreed. Since CK can get to 48 inches in the wild a 180 gallon tank would not work, just as a grow out. Oscars should get along with the clown knife as long as you get them both under 5" long. Here are some tips on keeping the two, Make sure there are lots of hiding spots so the fish will not be stressed out. ( Make sure you have some dark areas as well ). You do not want to feed the fish feeders since they can carry parasites that will block nutrients from going into the fish, and later kill it. Some great foods are Hikari, new life spectrum, trout, bass, tilapia, silversides and shrimp. When the fish get 12 + inches you should be willing to get a 500 gallon tank.
Some other fish that could go in the tank are.
- Rays
- Gars
- Bichirs
- large mormyrids ( mormyrus longirostris )
- Large cichlids ( PBass
- Pacu
- eel
- lungfish
- african arowana
- Datnoid
- Large silver dollars