I guess its my turn to be the bad guy again.
Most South American cichlids get too large for a 40 gal, except the dwarf type.
And a 40gallon is too small for 1 arowana.
So the pH doesn't really matter, if you put some normal size S American cichlids and an arowana in that size tank, my guess is most will be dead within 6 months.
So just how fast do Arowana grow? It was my understanding that they respond in growth to the size of the environment in which they are placed. So an Arowana placed in a 40 gallon tank will not grow as large as one living in a 300 gallon tank nor as fast, I could be wrong, it has been over 40 years since I kept Arowanas.