Strategy

Think in slots, not fish

The tank limit is the real resource in Leviathan. Every fish takes one slot whatever its size, and a full tank stops your drops, your eggs and the free Fry. Two numbers follow from the rules:

A dropped fish is worth 2.6 Fry on average (it is a Fry 40% of the time, a Neon Tetra 30%, a Clownfish 20% and an Angelfish 10%).
A free Fry is worth exactly 1.

So when slots are scarce, your drops fill them with more value than waiting does. When slots are plentiful, the free Fry are pure profit, and they only arrive while there is room. The healthy rhythm is to drop until the tank is nearly full, let the merges catch up, and drop again, rather than sitting at the limit.

That is also why the first Bigger Reef level, at 400 pearls, is the best early purchase. Two extra slots are a 14% bigger tank for the price of a few dozen merges. The levels get steep after the third, so after that weigh each one against an egg.

Drop where the twin can see it

Fish merge on their own once one sees the other, and a fish sees in a wide cone ahead of it plus a short circle all around. A dropped fish dashes off in a random direction for a second or two before it starts looking. The best you can do is give that dash a good start:

Glance at the preview, find the nearest fish of that species, and drop straight above it.
Shoaling Fry and Neon Tetras bunch up with their own kind, so dropping onto a shoal is reliable.
A Clownfish keeps returning to its own home spot. Drop a second Clownfish above where the first one keeps hovering, not where it happens to be right now.
The 0.35 second drop cooldown lets you place two or three small fish into the same area in a burst. A tight cluster is how combos happen: Fry pair into Tetras, Tetras into Clownfish, all inside the 2.5 second window.

Feed the predators on your terms

From the Angelfish up, your own fish eat anything three or more species smaller, and the pearls they pay for a meal are tiny next to the value of what they ate. You cannot stop it, but you can steer it:

Drop away from hunters. A hungry predator locks on to prey it can see or anything within about 175 units; the giants lock on from about 300. Drop small fish on the far side of the tank from your biggest hunter.
Use the full belly. After a meal, a normal predator waits 8 seconds before hunting again, and the giants 4. The moment after a Lionfish or Koi Dragon eats is the safest time to drop small fish near it. The Anglerfish recovers quickly, so it gives you no such window.
Merge mid-ladder pairs fast. A lone Lionfish is food for any Manta Ray or Leviathan in the tank. Once you have a giant, pushing Lionfish on to Koi Dragon matters, because from the Koi Dragon up nothing in the tank can eat them.
Accept some losses at the bottom. Fry are free and keep coming. Losing them to an Angelfish costs you little; losing a Clownfish to a Lionfish costs four Fry worth.

Playing the Spikefish

Stay ready from the one-minute mark. No Spikefish can appear in the first minute of a session, or while the tank holds 10 fish or fewer.
Look at the warned edge. The shadow shows the side and height of the entry. Tap it as it glides in, while it moves in a straight line.
Use the stun. Later Spikefish take 2 or 3 taps. Each hit freezes it for 0.3 seconds and nudges it up, so the next tap goes just above the last one.
With two at once, net the one nearest your best fish. A Spikefish goes for the nearest fish it can affect, so look at what is closest to each.
Guard bloodline Anglerfish and Manta Rays first. A hit does not kill them, but they come back one species lower with plain genes, losing their bloodline and its double idle income.
Leviathans are permanent. Nothing can eat, kill or knock down a Leviathan. Your first one is banked for good, and the run to the second is only at risk below it.

Breeding bloodlines on purpose

The rules for inheritance reward a few specific habits:

Surge before plain merges, not bloodline ones. Mutation Surge only raises the chance for a pair where neither parent has a bloodline. Turn it on when you are about to make a lot of plain merges, such as a tank full of Fry and Tetras.
Two of a kind beat one. A bloodline fish merging with a plain partner keeps the bloodline 35% of the time; with a partner of the same bloodline, 70%. If you have a Golden Clownfish, a second Golden Clownfish is worth more to it than any plain one.
High rungs pay most. A bloodline prize grows 40% of its base with every step up the ladder, and a bloodline merge also triples the merge payout. A bloodline born at the Anglerfish pays well over a thousand pearls if it is Golden. Protect bloodline fish on their way up.
The Abyssal Egg is a breeding fish, not a jackpot. About half its hatches carry a bloodline, and the hatch prize averages a few hundred pearls, well under the 5,000 price. Its value is a Pufferfish or Lionfish with a bloodline already in it.

Spending pearls well

Pearl Rush only pays for itself in pearls on a big reef. It doubles idle income for 2 minutes. Paid with 2,000 pearls, it comes out ahead only when your reef already earns more than about 17 pearls a second, for example a 32-slot tank of Koi Dragons, or fewer fish carrying bloodlines.
Eggs save drops, not pearls. A Reef Egg hatches 4 to 16 Fry worth of fish in one slot, which is most useful when the tank is short of room or a lone mid-sized fish needs a partner.
Upgrades are lost at a new reef. Bigger Reef levels and leftover pearls do not carry over, so there is no reason to hoard before finishing.

Before you close the game

Offline earnings come from the fish in the tank when the reef was saved, at half their normal rate, for up to eight hours, and only if you are gone for more than two minutes. Bigger fish and bloodline fish earn the most, so the best time to leave is right after a run of merges, not with a tank full of loose Fry.

One exception: if the saved reef contains a Cosmic Whale, the game starts a fresh reef the next time you open it. Your Codex entries and achievements are kept, but the rest of that reef is not, so finish whatever you wanted from it, like extra Codex entries, before you leave.