This is addressed more fully during the Fishman Island arc, which I just got done watching. It's one of the weirder arcs, politically speaking. Prior to the arc it has been well established that Fishman are considered inferior beings. During the FI arc there is a group that are pretty much presented as "reverse racists" who draw inspiration from Arlong and "hate humans for no reason" even though there is a clearly established history of humans enslaving Fish/Mer people and treating them as lesser people, including a flashback during the grand reveal that they had no root cause for their hatred of humans.