My personal build of Fish always has prioritized, and always will prioritize tribal synergy. Optimal or not, it's just what I enjoy the most. As such, whenever my build deviates from the norm in any way, it is almost always for the sake of including more tribal payoffs and fewer non-tribal cards.
The Core Pieces
Lord of Atlantis
and
Master of the Pearl Trident
are the whole reason this deck exists at all. As more-or-less functionally identical two-mana lords that grant your whole board evasion, they are fast, powerful, and incredibly consistent. The double-blue in their mana costs is the primary reason most Merfolk builds are mono-blue. Each should be an automatic 4-of in every Merfolk deck in the format, giving you a total of 8 copies between the two.
Spreading Seas
is the next most important piece of the deck. While its main purpose is to enable the islandwalk from your lords to make all your fish unblockable, the utility this card provides is not to be underestimated. It can shut your opponent off of specific colors of mana, it can act as a removal spell against
manlands
, and it can also shut down
tron
and other problematic lands. Some people prefer
Sea's Claim
instead, but I consider the extra mana in the casting cost to be worth it for the ETB draw. Needless to say, this should be a 4-of in the mainboard of every Fish deck in the format, though it does occasionally make sense to board it out in game two (in a mirror match, for instance).
Aether Vial
is a fairly obvious inclusion if you've played other tribal aggro/tempo decks in Modern. Since almost every creature in the mainboard costs exactly 2 mana, keeping a Vial on 2 will help you to empty your hand in a hurry, and it only gets better in multiples. That it also effectively gives your creatures flash and helps you get around both mana issues and countermagic are two points which push this already powerful card into sheer absurdity. Automatic 4-of. Even if you've never played another Vial deck before and are skeptical of this card, you'll very quickly realize how powerful it is after only a few games.
Mutavault
is the last of the core pieces of this deck, but certainly not the least. The most notable thing about it is that, when it animates, it becomes a 2/2 creature with all creature types until end of turn. "All creature types" includes Merfolk, which means that if you've got lords on board, this land can often represent an insane amount of unblockable damage for the equivalent of only two mana spent (one to animate, and one lost to opportunity cost when it taps while attacking). Automatic 4-of.
The Utility
Benthic Biomancer
: Easily the best 1-drop available to us in mono-blue. Before Ravnica Allegiance, some lists were running cards like
Mistcaller
or
Cursecatcher
in the 1-mana slot. I tried both, and wasn't thoroughly impressed with either, so I was happy beyond words when this card was spoiled. Yes, the mana investment to adapt is clunky, but it has a tendency to help you break through mana flood/screw, and since you can adapt at instant speed, the threat of activation can be rather annoying for your opponent. I will happily run four copies until Wizards decides to print an even better 1-drop.
Silvergill Adept
: One of the only fish in the deck that doesn't need two blue mana to cast, which can be surprisingly relevant when you're stuck with multiple copies of
Mutavault
and only one blue source. Drawing a card on ETB while also being a solid body makes this a great creature for helping us stay afloat in longer games, or draw through mana issues. Notable that when you cheat it in via
Aether Vial
, you don't have to reveal a Merfolk from hand, but you still draw a card. Should be run as a 4-of.
Merfolk Trickster
: Besides the lords, this is one of the strongest blue creatures ever to grace the Merfolk tribe. Flash it in before attacks to slow your opponent down. Flash it in during your opponent's end step to tap down a blocker. Hold it up at the same time as a counterspell, and then cast whichever one you need that turn. Play it before a
Harbinger of the Tides
to bounce whatever your heart desires. Target a creature with a problematic ability to shut it down for the turn. The amount of utility this one creature provides cannot be overstated, and for that reason, you should always be packing a full playset.
Harbinger of the Tides
: One of the less popular inclusions in the deck, to be sure, but I like it as a solid tempo hit and a virtual 2-for-1. Sure, you'll probably never cast it with flash, but in a deck that has 4 copies of both
Aether Vial
and
Merfolk Trickster
, I wouldn't be too worried about that. I run a playset due to the synergy it has with
Merrow Reejerey
, though many lists only run 2, if any at all.
Merrow Reejerey
: Rarely absent from Fish lists, but also rarely a full 4-of, Reejerey falls into a weird middle ground of being obscenely powerful, but costing just one mana more than is ideal. Pumps your team, taps down targets for
Harbinger of the Tides
, doubles the effectiveness of
Merfolk Trickster
at stopping attackers, can untap lands and Vials to help you play more Fish in a turn, and also has some extra utility such as tapping down Tron lands during your opponent's upkeep. I play it as a playset, but lists frequently run as few as 2.
Force of Negation
: Turns out that a worse
Force of Will
is still a supremely powerful card. Most Merfolk lists (and hell, most blue decks in general) are running a playset, and I can't say I disagree with that decision. Don't hesitate to board the whole set out against creature-heavy decks, and consider options like card:Spell Pierce(XLN) instead if control and burn are big in your local meta.
Waterlogged Grove
: Since the printing of Modern Horizons, almost every deck in Modern is running some amount of these new "canopy lands," and I don't think Merfolk should miss out on the action. This is more or less just insurance against flooding that also comes in untapped, taps for blue, and doesn't die to
Choke
. Sure, the one life to pay whenever it's tapped is annoying, but realistically it won't make a difference most of the time, whereas flooding out definitely will. In a worst-case scenario, this can even just be effectively cycled for 1 mana to get a dead draw out of your hand in the late game. I'll definitely be playing four for now.
Finally, nonbasic lands like
Oboro
and
Minamo
are there just to act as islands that aren't actually islands. Realistically, you could just run basics here, but so long as cards like
Boil
and
Choke
exist in the format, it's better to play it safe and not have a mana base composed almost entirely of basic islands.
Cavern of Souls
works here too, but I find that the "can't be countered" clause is almost never relevant, while not being able to spend the blue mana on countermagic is relevant rather frequently. It's usually safe to hover around 20 lands, and remain in the ballpark of splitting basics and nonbasics 50/50.
