I have often wondered that myself. Just googled them both. These seem to be the best definitions.
FISH
A weak player who loses money.
A player with a habitual losing record is considered a “fish”. Comparable to the idea of playing the children’s card game “Go Fish”, where the only way to win is to continually see/pull more cards from the deck with the hope of eventually outdrawing an opponent. In the long run, this strategy in poker will end in failure and a player's
bankroll will disappear. “Fishing” for scarce cards on a regular basis will label a player accordingly.
While a “fish” might come across as “a genius” for any given hand when they catch a miracle card, the “sharks” welcome them into the tank.
A fish is just one of the many negative slang terms people use to describe poor players. Other names include, donkey, donk, muppet, idiot, sucker, etc.
DONKEY
The poker donkey has replaced the poker fish. In the olden days of poker (before 1999) the bad players were called fish. Now a days they are called a donkey.
A poker donkey is a bad player, or is he? A poker donkey is a gambler. Not just any old gambler though. The poker donkey will hit their gut shot straight, seven high flush, and miracle full-house after a fourth street flush. The poker donkey is a terrible gambler. The poker donkey will gamble with incorrect odds and hit their miracle card.
A poker donkey does not know any better. Donkeys like to play and they will get paid at your expense. You can expect to find a poker donkey at the lower limits of poker. Usually they will go on good runs of cards. The donkey will run so good that they will be mistaken that they are actually good at poker and berate other players to their fine playing skills.
But, like all donkeys they finally end up on the rail watching other donkeys play. The poker donkey will often be mad and sulk and blame the cards for the absolute terrible run of bad fortune. It is only then that the poker donkey will dig deep to gain knowledge of the game.
THIS ONE I LIKE
FISH - A poor player. One who gives his money away. It's a well-known (though not well-followed) rule among good players to not upset the bad players, because they'll stop having fun and perhaps leave. Thus the phrase, "
Don't tap on the aquarium."