Every person will have a different answer to this question.
I find 27-28 degrees celsius (80-82.4 fahrenheit) to be around the most comfortable for the fish. When the temps rise into the 30 degree c + (86+) they seem to eat less.
I also like this temp because it allows for + or - 3 degrees safe zone that can happen with water changes and still keep the arowana in it's safe temp zone.
Every person will have a different answer to this question.