
All 50+ fish types have unique spawn conditions and abilities. Fill the tank by meeting their spawn conditions intentionally, or learn to ropes as you manage whatever comes your way.
Start each run with a school of Guppies. Grow their population to trigger various spawn conditions. Balance your predator and prey population and set yourself up for the next boss wave. Various fish types will attack bosses for you, and you can help nudge them in the right direction with a variety of items and combat abilities.
A properly set up tank can function autonomously. Zen mode and the Virtual Tank can also be played for a peaceful experience without the focus on bosses.
Catch fish, including rare variants to add them to your Virtual Tank collection and earn a special currency that can be used for permanent upgrades or to elevate your run.
There is always more to learn in Flounder. Poppy is spawned by manually popping bubbles. Her ability, when used to the fullest can propel your early game to new depths. By using her to spawn additional bosses, you reap all the rewards that comes from defeating them: you force the shop to open and drop a wealth of coins. With bubbles constantly spawning, you can push your micromanagement to the limit, leading to a build with several extra items. If you can carefully draw the outline of any fish type, she will spawn it. Beware, there is no margin for error in witchcraft.
In the same vein, Koy can also earn you extra items by forcing the shop open and dropping high value coins. The problem is, Koy are prey for many of the predators in the game, and protecting one until fully grown can take careful ecosystem management. Koy are spawned by letting coins fade out on the bottom. Leaving every bronze coin to trigger a Koy spawn is a very viable strategy. This means any fish or item designed to collect coins for you can actually prevent Koy spawning, limiting your potential. If you are able to fully grow a breeding pair however, you can eventually have an entire school of Koy, dropping coins and forcing the shop open consistently.
The many different item effects can range from helping you manage the ecosystem and bosses, adding UI elements or adding deep systems and mechanics. Some items might fit your playstyle so well, that you never want to play without them. That is where the loadout system comes in. Earn fishing points by catching high value fish in the main game, and spend them to purchase items to begin with every run.



Gacha for example, helps you control the chaos. If you are overwhelmed by fish spawning immediately when their spawn conditions are met, you can use the item Gacha to limit your tank once it has reached a population of 25. Right click a fish to pop their bubble and remove them from the queue.
Poseidon's Heart gives you the power of a god. It allows you to control the currents by holding right mouse button. Using this, you can force entire groups of fish to attack bosses, position your fish and separate them precisely, and combine with other item or fish abilities to bring them to the next level.
Unlock rare Pet versions of fish that never hunger or die. Use this feature to always have your 3 favorite fish by your side. Hammy (pictured above) is a great choice when paired with Poseidon's Heart. Normally, you would have to manually launch Hammy into bosses to make use of its ability, but with Poseidon's Heart, you can sweep all your fish toward bosses to unleash them all, including Hammy's devastating and stunning strike. Be careful though, all shark types can cross breed and if not kept in check, they will turn your tank into a feeding frenzy. All shark types can sense blood in the water and become hungry faster, unless you sweep away the blood pool using Poseidon's Heart or another similar ability.
150+ unlocks with many more planned.