Who/what is the fish?

The fish = the person that funds that system. It's his money that fund all of the other entities in that system: the gov, the house, the pro.

The fish is not to be viewed as "the sucker" or "fool". People often make that confusion.
It often happens that he is but it's not a requirement.
The better you know the details of the system in question, the easier it will be for you to identify who the fish is.


In poker the fish can be a rich business man that wants to have fun gambling. He doesn't need to be a fool or a sucker.
He is spending money at the tables like I would be buying a cake because I enjoy it.
I am the fish in restaurant biz/system: I pay a high price for food(compared to how much it costs to make it) but I save time which is more valuable to me.
I could make more $ saving that time than wasting it to cook. I would have to be an idiot to cook myself instead of play poker during that time.
The motives for being the fish are not important. The fish is still the fish even if his motives make perfect sense.

It's also likely that if the fish would not play poker, he would spend that money for something even more damaging to himself like alcohol or drugs. He is in a way "saving" himself money by losing only that amount by playing poker.

The good driver who in a previous post I identified as the fish in the car insurance system: maybe he is aware that he's likely to lose money by being insured.
But let's say he works in transportation, to risk losing his car would mean his whole business is at risk. It's still in his best interest to be insured because of him having so much more to lose.
All of that is again irrelevant: the point is, in the car insurance system he is the fish, regardless of his motives or his gains from a totally different system.

So fish= the customer? 
NO! 
The bad driver is a customer too but he is not the fish!
The poker pro is a customer but he is not the fish!

It's wise for all parties involved to respect the fish.
In poker the house gives him bonuses often. Pro players are ideally nice when talking to him.
Stores may give discounts to loyal customers(fish).
Car insurance companies give bonuses to drivers who have a good track record without accidents.
It's not about being nice or kind, it's just smart business to have that attitude towards the fish.