just a side note but are you aware you will need to upgrade your tank in the future for the arowana to more like a 350g minimum.
any of the earth eater cihclids (geophgus would make a good addition, my favourtite being the suriemensis. you could also try sevrums and oscars, bichirs, eels larger catfish.
depending on what size your fish are at the moment though you might have troubles introducing new smaller fish to your bigger fish.