6.5-7.5. It applies to silver(south America), red green (Asia ) , gold(Asia or Australia ) arowanas. Probably not suit for African arowana. But aro can tolerant higher like 8. Staple ph is more important as long as ph can be tolerated.
dont try this at home : but i myself noticed my fish became super active and eating alot when the water ph went down to 5!. when i raised it up to 6.5 it became lethargic and not as active. lol. mine's a rtg