I have pool pond with the water around half full dimensions 8.5 by 5.5 by 2.5All of these fishes are suspect especially cause your keeping them in a 55g with plans to upgrade
ThanksLike everyone has said anything we respond with is just a guess but I would guess Pacu. They are nippy and fast as lighting especially when they are young.
Plus they don’t mind swimming in the upper section of the water column where you will usually find an arowana.
It could be the gar, that would be my second guess.
Especially if it’s just a one time occurrence. A gar will take a swipe at another fish that is in its face but usually want stay after it unless it’s prey size.