|This article has not been revised for the current version (U34-476542). It was last updated for LU-356355. It may contain inaccuracies.|
The Pacu spends its time floating around in the pool of polluted water in which it was created. They will not fight back against your dupes and reproduce slowly, usually laying a single egg before expiring, which occurs in only 25 cycles. Pacu can only be captured and moved in an aquatic fish trap, which complicates domesticating them early on in the game. Alternatively, you can steal their eggs by telling your Duplicants to sweep them & setting up a high priority storage container in your target location.
Once placed in a tank of polluted or clean water, feeding from a Fish Feeder will slowly domesticate the Pacu over roughly 10 cycles. Its feeding and reproductive behavior drastically change once domesticated. It begins gorging itself on Algae or seeds, and rapidly laying out eggs (one roughly every 1 1/2 cycles) and defecating. This continues until the tank becomes overcrowded, at which point the Pacu returns to its typical behavior. This means starting with a single Pacu, with enough time to become domesticated, will result in an extremely large population of Pacu very quickly.
Since their room size is calculated based on the body of water around them, and there's a possibility of separating water tanks with mesh tiles, the overcrowed room size might be the same as the adjacent body of water. A similar problematic behavior arises if they lie in mixed liquids. This can also prevent them from reaching places (even withing the same liquid and room), or feeding from the fish feeder. A simple way of avoiding this is making sure the body of water is not connected (not even with mesh tiles) to another body of water and that there are no mixed liquids.
Pacu require more than 350 l of liquid in a tile to swim; below that, the critter is considered to be flopping, and can visibly be seen to flop about on the ground. Even though the description reads: "Fish out of water" when the Pacu is flopping, it suffers no health loss from it, being able to essentially flop for its full lifespan.
The Pacu has two additional variants. It is a temperature sensitive critter, and, provided it is placed in a pool of the correct temperature, will produce its variants consistently and quickly. They all maintain the Pacu's feeding and breeding behaviors, keeping all 3 variants tied for the most reliable source of eggs in the game.
The classic pale green Pacu, while slightly superseded by its variant morphs, is still extremely useful as a food source. If this is your primary reason for farming Pacu, you need not look any further than the original. They will deplete your algae quickly, so keep an eye carefully on your reserves.
The colorful Tropical Pacu is the first, and most frequently spotted, variant morph of the Pacu. They emerge when a Pacu reproduces at a temperature above 35 °C. They are a useful source of Pacu Fillet and eggs, and also provide a +25 decor out to a radius of 5 squares, making it strictly an upgrade from the Pacu. Effectively utilizing this decor bonus can be difficult however, as it only penetrates Window Tile, similar to the decor bonus of a Shine Bug. It does, however, mean outfitting your barracks or mess hall with a fish tank floor (or roof) will be rewarded by the aquarium loving dupes (and hammer home the "moon base" vibe).
The blue gulp fish is the second variant of the Pacu. They emerge when a Pacu reproduces at a temperature below 5 °C, and continue being a useful source of Pacu Fillet and eggs. They also have the drawback of requiring temperature management - too hot and they die off, too cold and the pool they are in starts to freeze. They do, however, convert polluted water into water, at a slow and consistent pace of 200 g/s. A large pool filled with these critters can produce enough fresh water to support a base... though water needs to be siphoned off quickly thanks to its higher freezing point than polluted water. Alternatively, if the polluted water is cold enough the water that the Gulp Fish produces will instantly freeze into ice, which can be collected and melted.
A newly-born tame Pacu will live long enough to lay an egg, even when not fed and in overcrowded conditions. With the population being sustainable by default, it is possible to rapidly increase it to an arbitrary size by feeding even a single non-overcrowded Pacu and separating the eggs into another tank. The resulting Pacus can be left on their own while they passively produce free eggs and Pacu Fillets from their life-cycle.
It is possible to sustain a single Pacu-breeder via Algae Distiller-Pufts pathway. One Pacu requires 140 kg of algae per cycle. This is slightly more than the output of a single Algae Distiller at 100% uptime. The 140 kg of algae require 420 kg of slime per cycle, which can be covered with some excess by 9 tame Pufts. These in turn would need 450 kg of polluted oxygen / cycle. Off-gassing such an amount from polluted water would require a surface area of around 188 tiles (Puft will require at least 144 tiles on their own to not get overcrowding effects). It can be supplemented by PO Vents and Morbs, although Morbs' production rates are inconsistent.
With 2 consistently fed Pacus it is also possible to maintain one tame Pokeshell.
Pacu Farming is more feasible if you can find a way to heavily optimize the system and reduce the algae consumption. Otherwise it will rapidly deplete your algae reserves. With automation and by limiting the amount stored in the Fish Feeder it is possibly to feed a single Pacu only a few kilograms of algae per cycle. The Pacu will eventually starve, but until around age 10 it will live of its internal energy store and continuously lay eggs. After that it will intermittently starve, but still have its reproduction increased for a short time directly after a feeding. The length of the starvation and with that the amount of additional eggs can be adjusted by choosing to feed it more algae. The number of produced eggs is thus less than if fed the maximum, but the far lower amount of algae used makes this an economical tradeoff.
The Pacu's short lifespan and relatively large food output makes wild Pacu an easy source of food. Place them in a tank of any size; even a one tile pool will do. They will produce one egg throughout their life even when overcrowded and untamed, keeping their numbers stable, while dropping fillets for your Duplicants to eat when they die, with no resource costs.
An easy way to get Pacu into a tank is to move their eggs instead, via an Automatic Dispenser, a Conveyor Chute, or by moving the eggs to a Storage Bin in the pool and then unassigning storage from the bin, causing the eggs to drop.