A pathfinding bug for fishes that occures in cases if the "fish entity" enters into a non full, waterlogged block (fences, iron bars).  This affects both solo fishes or individuals inside schools.

I think it's work like this: 

• As the basic phatfinding AI leads the fish entity into a block it tries to lead it into the exact center of that block.
• If that block is a full water block there is no broblem at all.
• If the block is a waterlogged fence or iron bars, the fish can't get into the center, instead it's goes into some kind of "Im on my way there" state.

 If the entity once got into such waterlogged block, it will never tryes to move again. As soon as the player destroys the fence or iron bars, the fish immediately starts to move again freely.

My opinion is: The pathfinding fish entity is getting itself stuck in a never ending "i want to go there" loop if it can't enter into or pass trough the center of a water block. 

EDIT:

        This thing occures most of the times if the player goes away and returns to the area.. As I see most of the times it affects fishes that want to go beyond the actual waterloged block cause if the block is destroyed they immediately start to move at the direction they was facing.

   It's just a speculation, but I think to correctly develop the bug the player must go away from the fish (out of the fish AI proicessing range or even put the fish into a lazy cunk) once it's looks like it's stuck inside the waterlogged fence or iron bars block. When the player returns the fish AI somehow forgets to guide fishes wich are "stuck" in such blocks.

To reproduce this:

 - make a pool, and create some waterlogged fences, iron bars, stairs etc. inside it.

• Put in some fish
• Wait and see.
• IF you see a fish got "stuck" in the waterloged block go away from it.
• Return and the fish will not move itself ever again until you destroy the waterlogged block.