There are many more people on here who can answer this much better than I can, I can just lead you into a direction, but more times than not, white feces from a fish is caused by some sort of internal parasite, which affects the digestive tract system causing poop to go from brown/black and somewhat solid to stringy, less condensed and white. I’m no experienced arowana keeper but this is just my two cents on that matter. Good luck!
Ps: please just test water, yes maybe it’s perfect but still just in case it never hurts one to test their own water.