The game simulates a fishtank with 2 Black Mollies (hence the name) and a number of "machines", which are harmful to fish. Each player has four fish and the objective is to keep them alive as long as possible. The game board is variable and made up of 24 cards. The roll of a D4 die decides about the machines being turned on or off or the current in the tank being activated. If a fish ends its turn on a card with the Black Molly or an activated machine it is taken out of the game. The last player with fish(es) on the board wins the game.