Fish Hell is the 1st stage of The Crazed Fish. Next stage: Ogre Island.
The Crazed Fish Cats aren't the boss of Fish Hell. Each is a real threat (87,000 HP, 260 range, area hits that can crit), and they keep coming in groups of three and four all fight long, but a level 30 Crazed Bahamut clears them. The boss is the Super Metal Hippoe that comes out when you touch the enemy base. It's Metal, so any hit that isn't a Critical does just 1 damage. That makes this stage a gear check before it's a skill check: you need a Critical hitter, and a Crazed Bahamut strong enough to delete the Fish while the crits work on the Hippoe.
The lineup that does it
Nine units and one open slot, no gacha. Five meatshields: Eraser Cat, Samba Cat, Macho Cat, Crazed Wall Cat and Crazed Macho Cat. Five is enough here. The Critical hitter is Major Space Cat, the free one, which drops from Attack on R-Cyclone (there's a guide for that stage). It attacks from range and crits rarely, but often enough. Level it as far as you can: every level means fewer crits needed on the Hippoe. Dragon Cat stacks at long range and chips any Fish that walk up past the Hippoe. Valkyrie clears the Crocos. Crazed Bahamut does the real killing, and it has to be level 30. At level 20 it doesn't kill the Fish fast enough, and you get overrun whatever else you do. Use the open slot for anything useful you own.
Items
Bring Rich Cat. From the first second there's no break, and without it you spend most of the battle short on money and struggling to level your Worker Cat. Cat CPU gets you to the point where the battle plays itself. Sniper the Cat is worth it too, because its main job is knocking the Hippoe back when it bears down on your army, and Metal doesn't stop that. It could occasionally push a target out of Bahamut's shot, but Bahamut is firing into a group of Fish, so it rarely matters. The stage can be done without any of the three, just harder.
The start: pile up the Crocos, then one blast
The Crocos come immediately and keep coming. Don't pick them off one by one. Let them walk up to your base and bunch into a mound. Put a few meatshields out to hold them in one spot, then send Valkyrie. Her area attack kills the whole pile at once and pays you for all of it. Keep meatshields in front of her so the first Crazed Fish doesn't reach her, and save for Crazed Bahamut. When he's affordable, send him, and he takes out that Fish.
the Croco mound building up at your base · plays here, from that second
Keep the fight on your side until you're rich
From here the Fish come in groups. Meatshield a little more, but don't overspend. You're saving, and touching the base releases the Hippoe plus three more Fish at once. So keep the battle on your half of the field. Once you're in five figures, turn the CPU on. From there Bahamut's kills on each group of Fish refill your wallet, and the run mostly plays itself. You only run short when Bahamut misses a couple of shots or spends one on the Hippoe instead of the Fish.
Valkyrie clearing the pile, the Bahamut save starting · plays here, from that second
The Hippoe, and the clock
Once the Hippoe is out, it's a waiting game. Sniper keeps it off your army, meatshields keep it busy, and Major Space Cat's crits wear it down. Don't let it drag on, though. At 16:07 an Assassin Bear comes out: fast, huge range, and it wipes your units. It has very little HP, so if a run has gone on that long, turn the CPU off and have the cat cannon ready. Fire it the moment the Bear appears, and it dies. With Major Space Cat you'll usually finish well before then. The Bear mostly catches runs relying on a weak crit, such as Island Cat's 2%.
the clump at the base, the CPU running it · plays here, from that second
After the Hippoe
Once the Hippoe falls, only a few Fish remain, and the base goes down behind them. The reward, Crazed Fish Cat, becomes Crazed Whale Cat when you upgrade it. It's one of the most useful Crazed units, and it's in the clears of Festival Gross and Berserk on this site.
the crits grinding the Hippoe down · plays here, from that second
Other Critical hitters
Any Critical hitter works here. Catasaurus (Jurassic Cat's True Form) is the Rare Capsule pick the wiki names, and any Uber or Super Rare with a Critical backs up Space Cat. Moneko also works if she's what you have, but she's the weakest option, not for lack of crits: she has almost no range, attacks slowly and recharges slowly. Paris Cat's range is excellent against the Crocos and the Fish behind the Hippoe, and Thief Cat makes the start pay better, but neither is needed.
THE SHORT VERSIONYou need a Critical hitter and a level 30 Crazed Bahamut. Pile up the Crocos and blast them with Valkyrie, save for Bahamut, and keep the fight on your side until you're in five figures. Then turn on the CPU and let the crits grind the Super Metal Hippoe. If the clock nears 16 minutes, hold the cannon for the Assassin Bear.
The general approach, from the wiki's strategy notes. The guide above walks through one full clear.
Super Metal Hippoe is the real boss — do not attempt this without Critical hitters.
- Catasaurus from the Rare Capsule, or the anti-Red/Floating units that clear Crimson Catastrophe for Space Cat, supply the crits.
- Beyond that a generalist lineup works: a few meatshields plus strong, spammable Area Attackers for the Crazed Fish.
ERASER CAT S Wall in the guide's clear. 
SAMBA CAT D Wall in the guide's clear. 
MACHO CAT D Wall in the guide's clear. 
CRAZED WALL CAT S Wall in the guide's clear. 
CRAZED MACHO CAT A Wall in the guide's clear. 
MAJOR SPACE CAT B Critical in the guide's clear. 
DRAGON CAT B Ranged chip in the guide's clear. 
TRUE VALKYRIE CAT A Clears Crocos in the guide's clear. 
CRAZED BAHAMUT CAT A Lv.30, kills Fish in the guide's clear. ⚙ Trait counters worked out from the roster — the guide's plan doesn't need them; kept for reference
Croco is Traitless. Anti-Traitless units take less damage and deal more.
HITMAN CAT A Hitman Cat: Deals massive damage to Traitless enemies 
LONE CAT AND KITTEN A 100% KB + Freeze + Resist vs Traitless. Also fits “Out-range Croco.” 
DRILLER CAT A 100% Weakens Traitless to 10% and (True) Strong vs Traitless. Also fits “Out-range Croco.” 
D'ARKTANYAN S Strong Against Traitless on a Long-Distance area attack — answers the many Traitless bosses. Also fits “Out-range Croco.” 
JAGANDO JR. A Lord of Ruin Jagando Jr.: Strong against Traitless enemies (Deals 1.5x damage, only takes 1/2 damage) Croco reaches 110 range. Units that out-range it can chip safely without taking hits.
GROSS CAT A Range 350 — fires from beyond its 110. 
MASTER URIL A Range 405 — fires from beyond its 110. LONE CAT AND KITTEN A Range 300 — fires from beyond its 110. Also fits “Counter Traitless.” DRILLER CAT A Range 460 — fires from beyond its 110. Also fits “Counter Traitless.” D'ARKTANYAN S Range 340 — fires from beyond its 110. Also fits “Counter Traitless.” 
The castle you destroy to clear the stage.