This Magic: the Gathering variant is designed for two players who both play out of the same 80 card deck.

The deck contains ten copies of the signature creature for which the deck is named, the Dandan (originally from Arabian Nights, Magic’s first expansion), and eight Memory Lapse which are also represented in the name. There are four copies of Accumulated Knowledge, and two copies of all the other cards in the deck (aside from basic Islands), many of which manipulate the top of the singular library in some way.

RULE CHANGES

The game of Forgetful Fish is played like a standard game of Magic except for the following rules changes.

The two players share a library and a graveyard placed between them so that both players can reach them with ease. A card that refers to the “owner” of a library or graveyard or refers to “your library” or “your graveyard” refers to the singular library or graveyard.

Since both players share the library, the “owner” of a card, is the person who played that card from his or her hand.

Players will only lose life in increments of four, and therefore a starting life total may be represented by five counters (5x4=20) or tracked on a single six-sided die. Each time a Dandân deals its 4 damage to a player, he or she loses one counter, or one number off the die.

For starting hands and anytime both players must draw cards from the same effect, the cards should be dealt from the top of the deck, one player than the other, as though dealing a standard card game. The player to play first, or the active player deals.

Once the starting player is determined and the players have their initial hands, a player without two or more lands in his or her hand may reveal that hand to take a mulligan without being required to take one fewer card. The player may take free mulligans until he or she has a hand that contains at least two lands. If only one player takes a mulligan, after reshuffling that player takes the top seven cards of the deck as his starting hand. Players may also take standard mulligans by taking one less card each time, but never scry the top card of the library after taking a mulligan.

The stack works as normal, but can get quite complex. Players should allow time for their opponent to respond and make sure his opponent passes priority before beginning resolution of the stack, each time a spell resolves, and before drawing a card from the top of the library.

Optional Rules - Players may agree to any or all of these optional rules before play.
Finding Dory: Dandâns are considered every card type, allowing them to be retrieved with Mystic Retrieval or Mystical Tutor.

Fish Eyes: Any card placed on top of the library that both players are aware of (i.e. a spell countered with Memory Lapse or a Dandân that is Metamorphosed) is placed on the library face up.

In The Mind’s Island: Instead of using the free mulligan rule above, the players can remove four islands from the deck before dealing opening hands. Each player begins with two untapped islands in play, and play begins with the first player’s main phase. Each player begins play with an opening hand of seven cards as normal and the standard Magic mulligan rules are in effect (sans scry). This has the effect of a quick start and also forgoes the chance that only non-basic lands are played preventing Dandâns from attacking a player.

Remember the Fish:** If the deck runs out of cards, and a player is forced to draw a card, he reshuffles the graveyard into the library before drawing a card. Players cannot lose the game by being forced to draw a card from an empty library unless there are no cards in the library or graveyard due to exile from multiple Diminishing Returns.

Thanks for the Fish: A card that refers to the “owner” of a card, is treated as referring to that card’s “controller” instead. This allows Ray of Command, combined with Unsubstantiate or Supplant Form, to steal a Dandân on a permanent basis.

Strategy
Control the draw, control the game. Because both players draw from the same deck, control of the top of the library is key. Take the best cards for yourself and give your opponent every land you can spare. At the same time, make sure you have enough lands yourself. Only Memory Lapse the cards you cannot deal with another way or will help you more if you were to draw them.

It might not be obvious the first time taking a look at Forgetful Fish, but the key to killing a Dandan is to either make it so your opponent does not control an island (Vision Charm) or changing the text of the Dandan to have it sacrificed if its controller does not have another basic land type (Crystal Spray or Mind Bend). Note that Vision Charm will kill all Dandâns and only Memory Lapseing the Vision Charm, bouncing the Dandan or Vision Charm to hand, or using Dance of the Skywise can save a Dandan from this effect. Changing the text of the Dandan with Crystal Spray or Mind Bend in response or after the sacrifice trigger goes on the stack will not save Dandan from being sacrificed when the triggered ability resolves, as it is not an "intervening if" trigger (Rule 608.2a). Note also that Mind Bend, Crystal Spray, and Vision Charm’s choices (other than mode and target) are chosen upon resolution and not upon their casting. This makes it pointless to try to avoid a Dandan sacrifice by changing a single land’s type or changing Dandan’s text before the text or lands changed are chosen on resolution. The player could just make a different choice.

If your opening hand contains a couple non-basic lands, you might consider not playing an island for a few turns so that Dandans cannot attack you and you can collect cards that will give you a good start when you start dropping your own Dandans. You likely will not want to miss a land drop to continue this short term strategy.

Card advantage is a big key and control of the Accumulated Knowledges is just as important as Dandans and Memory Lapses. Unless you desperately need to draw the top card of the library, you’ll likely want to wait until there is at least one Accumulated Knowledge in the graveyard before you cast the one in your hand. If you significantly lose card advantage or have no other option to save yourself from damage, try to reset the hands with a Diminishing Returns.

Expect a Memory Lapse. If you don’t have the mana available during your opponent’s end step, It is sometimes beneficial to cast an instant spell during your upkeep simply so that if your opponent Memory Lapses it, you can draw it again on your draw step. Mystical Tutor right before your draw step is sometimes beneficial in this way also. Likewise you’ll most often want to wait till after your opponent’s draw step to cast an important spell just incase he uses Memory Lapse.

Unless your opponent taps out, Unsubstantiate is seldom well played on your opponent’s spells on the stack if they can just recast it. Instead save it to bounce a blocker, save your Dandan from destruction, or better yet, return your own spell that is about to be Memory Lapsed, and cast it again.

Try to save Accumulated Knowledge, Brainstorm, and cycling lands for when you or your opponent knows what card is on top of the library and you want it, likely don’t want it, or don’t want your opponent to have it. Predict is also best saved for moments when you don’t want your opponent to draw the card he has set himself up to draw. Vision Charm can also be used in this manner, but is often more useful as a board wipe.

If the game goes long and players fail to successfully cast a Diminishing Returns (or if too many are cast), it may come down to who can make sure they have a card to draw when the deck is depleted. If the deck is nearly out of cards, make sure you have a trick or two to place a card on the library when you are forced to draw. Memory Lapse your own spell if need be.