for a good sized tank for arowana, your gonna want something thats long as well as just as deep for their length, as well as the clown knifefish's length. id go with the size tank thatd im gonna get one day, an 8x3x2 ft, or 360 gallons. with this setup you could do three clown knives in that tank kinda like.
1 silver aro
1 oscar?
1 JD? (both can be slightly upped with the right filtration)
1 stingray
3 clown knifefish
the brute of your stocking will be the clown knifefish in this setup, hence only one arowana and a few smaller tankmates such as the oscar, JD, and stingray. now i said slightly upped, meaning possibly two, wouldnt do that for teh JD though, but oscars are easy to add more.