Many people keep their arowana in high pH without problems. 7.8 is not that high, add driftwood or peatmoss to lower the pH. This could lead to disaster though, best bet is just to keep a stable pH. You can add R/O water to lower your hardness.
I would just do some waterchanges and leave the aro alone. No much else you can do, the oscar may have just chased it to death.