The Deck
Benthic Biomancer plus Deeproot Elite is good. Kumena plus any other two fish is good. Merfolk Trickster overperforms again and again. Your threats may be small, but they are all capable of turning lethal if not dealt with. Combined with the added tools of Breeding Pool (20,000 leagues better than Botanical Sanctum) and the easy splash of Collision//Colossus for a spell that deals with the only large creature threats we care about while still fitting into the early game and creatureless matchups, I'd say Fish has never been more on the menu.
Basically, drop some early threats and make sure they stick or that your opponent has to incur card disadvantage to get rid of them, and you should be set. Mono-Blue has proven that tempo has a powerful presence in this meta.
With this deck, you can play control against aggro, aggro against control, and midrange against tempo, as need be. If you don't believe me, test it yourself--the number of lines of play rarely leaves anything to be desired. Remember: Merfolk Trickster can be played well at the beginning of either player's combat, mid-combat, and the end of your opponent's turn. You might even Main Phase it now and then.
Why Merfolk instead of Mono-Blue?
To be perfectly honest, half of the appeal to me is... I just like them. I don't like playing the same thing as half the people at my LGS. I like playing decks with cute and/or clever names and perhaps a theme or gimmick.
The other half is legitimate: while the deck is not as consistent as Mono-Blue, it is more flexible. You can go wide in a much bigger way, which makes this deck have a more favorable matchup against red than blue. As soon as you take Tempest Djinn out of the picture and you're free to run whichever lands you want, the Gruul pieces are doing a TON of work. Having additional combat tricks to Merfolk Trickster in the forms of Swift Warden and Collision//Colossus makes your opponents blocks much more hesitant--something that can cost them the game.
Plus, you know what's almost as fun as Entrancing Melody-ing a Hydroid Krasis? Entrancing Melody-ing a Jadelight Ranger or Mist-Cloaked Herald for tribal shenanigans, then killing the Krasis with a Collision//Colossus.
Why Red??
First, given the presence of shocks, letting your fish do a little splashing has never been so easy. If you're asking why we're bothering with a third color at all, just try playing in this format with Incubation//Incongruity being your only permanent response to a Hydroid Krasis. The question is which color you want to splash.
The argument for black is the flexibility of Status//Statue and decent recursion to play the long game against control. To be honest though, I'd much rather have the tempo of Collision//Colossus--two mana to kill a Krasis is much better than four and miles better than one plus a trade. Domri's -3 does the same work as a one-off piece of recursion while his +1 lets your threats swing in the turn you play them.
The argument for white is go-wide of Radiant Destiny plus the defense of Shalai. But the best part of Merfolk is the flexibility; giving up that to go all in on going wide makes the deck weaker to Deafening Clarion. Plus, while she's definitely good, Shalai is a sorcery speed four mana non-Merfolk creature that doesn't even draw you more cards. You've already got enough doing her work in this version with Swift Wardens and Kumena.
