Fish is built entirely with Blueprint.
As far as pathing- these fish will swim to random Water Targets and avoid things in their way.
If you need them to follow a specific path of Water Targets you could set that up relatively easily.
instead of using a random Water Target you could set it up to have the Water Target tell the Fish where the next Water Target is (As a variable). or something along those lines.
Unreal doesn’t yet have a standard navigational system for ariel movement (non ground based) so we are stuck with node based movement for now 
I’m using blocking volumes to keep the fish contained where I want them.