Fishermen fish for fresh, fat, and colorful fish — and if you feel like it, you can join in! In Fat Fish, each player is out to find fat fish of different colors. The fatter the fish, the bigger the haul, and the more colorful the catch is, the more successful the fisherman will be. The player who fishes fabulously and secures the most valuable haul prevails over the other players and wins the game.

In more detail, the game contains one hundred fish cards in five colors, with the cards numbered 1-6. To set up play, shuffle the deck and deal each player a face-down deck of twelve cards, with everyone drawing a hand of four cards from their own decks. Place three cards face up to form the start of lines, then place another 3-15 cards (depending on the number of players) face down to form a reserve. On a turn, a player chooses one line on which to play, then plays 1-4 cards on this line, playing all the cards in a row from left to right. The color of each card played must match the rightmost card in this line. Thus, if the three starting cards are red, yellow, and blue, you can't play both a red and a blue card since you can play in only one line; you can, however, play a blue card onto the line that ends with a blue card, then play a green card onto the blue since no line has a green card at the end of it. At the end of their turn, the player refills their hand to four cards.

As soon as a line has five cards in it, the player who laid down the fifth card must take this line and end their turn. They choose one card of each color in the line and place them face up in their positive points pile, then place all remaining cards face down in their negative points pile. (If the line has five cards of one color, the player places two cards in their positive pile instead of only one.) The player then draws a card from the reserve pile to start a new line.

Once players have played all twelve cards from their personal decks, the game ends and players sum their positive and negative points to see who has the highest score.

As a variant, Fat Fish includes nine fishing license cards that give players bonus points, sometimes for being the first player to meet a certain condition, such as claiming a line that contains cards of only 1, 2, and 3, and sometimes for meeting a condition at the end of the game, such as having collected the most 1 cards in their positive pile.