The variant is somewhat akin to a board game as players do not bring their own individual decks. Instead, there is just one communal deck shared between the players. Forgetful Fish features an 80-card deck. The “Forgetful” part comes from the six copies of Memory Lapse, while the “Fish” part is from the ten copies of Dandân. The rest of the deck features a slew of blue instant and sorcery spells to take advantage of the top of the deck, the stack, and The Fish. The only effect that can deal damage in the format is Dandân.
Forgetful Fish is played much like any other game of Magic. Players start at 20 life, they draw a hand of seven, they discard at the end step if they have more than seven, and they can lose the game if they draw from an empty deck.
Players opening hands are dealt like a game of poker, one card for the first player, one card for the second player, and repeat. If an effect causes both players to draw a card, then the active player deals the cards in the same fashion as the opening hands, starting with themselves. Both players also share a graveyard. If an effect returns a card from “your” graveyard or library, it refers now to the shared graveyard/library. Also, the “owner” of a card is simply its caster.
Concerning mulligans, players may first take one or more free mulligans for hands containing less than two lands or spells, revealing any hand not meeting this criteria; then, once the previous criteria have been met, they may opt to take further mulligans for which standard rules apply.
https://mtg.fandom.com/wiki/Forgetful_Fish