Fish at the bottom of the tank looking poorly? You need to do some fundemental (things you must do!) things.
Test your water, if you dont know how or cant take a sample to your fish store.
Do a water change. Allways helps think 25%.
Add airiation to the tank, air stone or redirect your filters to 'rough' up the surface.
Check the tank temperature, might be cold??????
One last thing, arowana do chill at the bottom sometimes, especialy after a big feed.