Things that go wrong:
Checking behind - if you can't see a fish in front of you then you are the leader fish. If you check behind for a moment to see if everyone is behind you then in that moment you will lose the audience. The challenge if to take the leadership quickly without looking for approval or checking - "are you sure you want me to be the leader?"
Wiggling the fish - lots of people wiggle their fish, sort of doing mime fish. The sign of a fish. There are two problems with this. Firstly that isn't really what fish do - it is more what their fins do. Secondly, if you were following the rules as laid out above, every wiggle one way should change the leader. So you should now be a follower. So there should be no wiggle back.
Small movements - same as above really. Avoid them. We don't want to make a fish cocktail party which is what you will end up with. Fish chatting.
Big corners - The leader fish makes a big clear turn and everyone follows and at the end of the turn the leader fish is the same. The group is just following the fish. This might be directed if necessary and it might have a place in the show. But it doesn't need to be practised here now. It is too easy. Every turn should change the leader.
Everyone tries to get their fish to the front - Almost always when we first do this exercise people reach to be at the front and get their hands as close together as possible. But that isn't what makes the fish relate to each other. And it makes it difficult to move and difficult to see. Spread out. What makes the fish related is that they are swimming in the same direction. How far apart can your fish be and yet "swim together"?
What makes a leader is the followers. The back fish makes the size of the shoal of fish. Hanging back makes the shoal of fish bigger. If your fish was the front and now it is the back it is often surprising how long you have to wait before you can move, but you must wait or you will be crashing into the fish at in front of you (that was behind you). It takes time to make the shoal face the other way. You want to make this as fast as possible as a group. That means working together. Giving up the leadership and becoming a good follower is both psychological and technical.
Simon Says - this is another common mistake. When the fish becomes the leader they feel like they must do interesting this for everyone to copy. This is impossible. I've watched people lead the shoal of fish down to the ground, up into the air, do sign waves in the air, wiggle (see above). None of them work. None of them impress an audience. Why? Because they can see what you are doing and how you are doing it and when everyone copies one fish - well its essentially the same as having one fish.
The fact is there is nothing interesting to do! The only interesting thing a leader can do is to make another leader. To choose that moment and be as surprising with it as possible.