This is Archetype Analysis: Fish
Learning to Swim
Back during the "First Era" of Yugioh, the only Fish that ever saw use was Terrorking Salmon and 7-Colored Fish in ALO Beatdown. The rest were just spares that barely got to anything (Soul Eater, Flying Fish, Tongyo) by being just too weak to even try any kind of beatdown strategy during that era. There were other Fish released with effects, but they were mostly lackluster or not fitting any particular strategy. This was the bane of Fish for many years until we got a miracle in Tactical Evolution.
Tactical Evolution finally brought a meaning for Fish, and that was to cause destruction and have some sort of control. Cranium Fish was meant to destroy face-downs, Abyssal Kingshark dodged non-targeted destruction, and the others somehow just had a level of synergy between them. But all in all, it still wasn't enough to make a solid Fish deck. That is, until the godsend arrived a few packs later.
While it is true that Phantom Darkness brought the game a ton of powerful cards, there was one that was quite overlooked until now. It was where the world came to know of Coelacanth's existence as the boss of all Fish. However, the giant was here, but still barely had an army to lead. The deck by then became a simple swarm-control deck, using Coelacanth and Cranium Fish as the main attackers while keeping the field full of Fish for Golden Flying Fish to destroy. Little did people know that this king of fish was soon going to become a force to be reckoned with...
An Exclusive Ocean, How the TCG Boosted Fish
But even with the presence of Coelacanth, Fish were still feeling incomplete in their arsenal. That is, until TCG Exclusives began spitting out new toys for Fish. They first gave us our first tuner, Royal Swamp Eel. At an instant, the deck turned into an OTK deck just by being able to swarm 4 fish that gave access to 2 level 8 synchros and could go for 8000 damage. But it wasn't done yet, we got the ability to recycle our fish via Metabo-Shark and a way to speed the deck up via Nimble Sunfish.
Frogs and Temples, How Fish are Making an Impact
Once all our support finally arrived, we had a full school of fish at our disposal. And so, we created various versions, but only 2 have known great success: Froggy Fish and Mausoleum Fish.
Froggy Fish consisted in using the Frog engine (composed of 3 Swap Frog, 1-2 Treeborn and 1 Ronintoadin) to not only thin the deck, but also set up tributes for both Coelacanth and Monarchs alike. While it's a slower paced deck, it can be quite explosive under the right setup, but it's always keeping field presence and runs more like a Frog Monarch deck due to this.
And then there's Mausoleum Fish (my current variant). It's more of a control build, allowing for Mausoleum to grant access to cards like Vanity's Fiend, LADD, End of Anubis, and the usual Coelacanth. It tends to use cards like Snowman Eater and Thunder King Rai-Oh sometimes in the main deck to counter strategies directly from game 1. It's a whole lot more explosive than Froggy Fish, but allows for a larger degree of control. Currently it is the newest and fastest growing variant out there, but both Froggy Fish and Mausoleum Fish are strong contenders and shouldn't be taken lightly.
Here's an old video on my old Froggy Fish build, along with a tiny guide of how it plays out. Note: it's a build from the previous format.
And here's my current Mausoleum Build:
Vanity's Fiend x2
Gorz, Emissary of Darkness
Thunder King Rai-Oh x2
Royal Swamp Eel x3
Oyster Meister x2
Nimble Sunfish x2
Golden Flying Fish
Mausoleum of the Emperor x3
Moray of Greed
Pot of Avarice
Mystical Space Typhoon x2
Book of Moon x2
Gold Sarcophagus x2
Call Of The Haunted
Royal Decree x2
Extra Deck: 15
Shooting Star Dragon
Scrap Archfiend x2
Black Rose Dragon
Iron Chain Dragon
Brionac, Dragon of the Ice Barrier