I would also suggest a red wolf fish. They are the smallest wolf fish, but still get a good size (10 inches). They are incredibly active and a great show fish, as well as highly aggressive. One of the most rewarding fish I have ever kept. There isn’t much by way of tankmates that isn’t also a snack, but in a 55 it could possibly be kept with an armored catfish. Mine is in a 36 bow front - as he grows he may need more space but at 6-7 inches now he isn’t showing signs of needing an upgrade.
A senegal bichir is also a good fish for that size tank - it isn’t large enough to prey on other larger fish but is an impressive fish on its own. Delhezi get larger but are said to be good in a 55 (I personally wouldn’t do less than 75 for the extra width). A sen would do well with the African butterflyfish I mentioned in your first thread (which is a relative to silver arowana).
Personal preference, but any of these fish to me are far more appealing than a silver arowana (maybe because aros are overdone).