Stock the tank before the night kills it

Set the tank up in this order

1. Place the Recirculating Aquaculture System and give it power. Filters only scrub on powered nights.
2. Fill to the 12,000 mL cap before you add stock. Add the adult water needs in the table and make sure you are over that sum, not just over half of it.
3. Move every fish and every fry inside before sleep. The notes treat 'outside at night' as fatal for fish and as destruction for fry.
4. Load the Livestock Feed Dispenser. An empty stomach for two nights is its own death check, separate from dirty water.
5. If you want breeding, keep a fed adult female and a fed adult male of a tilapia line together. Eel, Electric Eel, and Sturgeon do not lay.

Gutter Tilapia also strips heavy metal, chemical contaminant, and physical contaminant from the tank water. Graft Tilapia yields one Tilapia Skin Graft per 200 g. Sturgeon in these notes is always female and is the caviar line. The Aquafarmer start hands you an adult tilapia pair plus one fertilized Tilapia Roe.

Troubleshooting: the fish suffocated, starved, or ate the clutch

Suffocation is a water-volume check, not a mystery status. Add up what the adults require. If the tank holds under half of that sum, the stock is wiped. Between half and the full sum, health falls with the shortfall: a tank at half the requirement leaves half of max health. Organic Waste is added by body weight every night. Damage begins when that waste is a 3% share of tank volume. An 8% share is lethal. Graft Tilapia takes twice the waste harm. Gutter Tilapia takes none. That is why a 'clean looking' tank can still wipe Graft Tilapia first.

Hunger is a second checklist. Healing is 10% of max health, and only on a powered night where hunger is at least half of max. Two nights stuck at zero hunger kill the fish even if the water is perfect. After the old-age day on the table, each night is a 50% survival roll. Growth nights require hunger of at least 37.5% of max. Under that, the age counter does not move. Under 75% hunger the fish pulls dispenser feed up to its target, then carcasses, then loose pellets or feed in pieces up to 85%, and other food as a whole item. It will also take the smallest live body at or under 20% of its own weight. Eggs and fry are treated as weightless in that hunt, so a starving tilapia clears its own clutch.

Feed choice matters. Omnivores take any food. Eel, Electric Eel, and Sturgeon take meat only, and meat in the diet also speeds tilapia growth. Industrial Fish Feed is 5,000 kcal and holds growth to 75% speed. Gutterflow Pellet is 4,000 kcal. If those pellets pass 25% of the diet, the cuts drop to Trash Grade (−75%) and stop counting as food. Gutter Tilapia is exempt from that pellet penalty.

What you can actually sell

Fillet with a blade, the Auto Butcher, or the Livestock Feed Dispenser. Calories out equal calories eaten times the meat conversion, and the biggest cut is taken before the smaller ones. If at least 25 kcal is left over, that leftover is Fish Scrap. An adult tilapia gives a Fish Fillet and a Tiny Fish Fillet. Gutter Tilapia gives a Fish Fillet and Fish Scrap. Graft Tilapia gives a Small Fish Fillet, a Tiny Fish Fillet, and Fish Scrap. An eel gives three Eel Fillets and Fish Scrap. An electric eel gives two Eel Fillets, a Small Eel Fillet, a Tiny Fish Fillet, and Fish Scrap. A sturgeon gives four Large Fish Fillets and a Small Fish Fillet.

Egg laying, for a tilapia female whose hunger is at least 50%, is a 15% chance each night and produces 6 eggs. A fed adult male in the same tank fertilizes them. The hatch is 6 fry, and those fry become 6 juveniles. A Tilapia Skin Graft stabilizes a wound at wound level −2. Unfertilized Graft Tilapia Roe does the same job at wound level −3. Roe eaten as food is 60 kcal.

An adult sturgeon adds 40 g of eggs per night, capped at 15% of stage weight, and only while health is at least 90%. One jar is 100 g. The Surgery Toolset pulls every full jar off a living sturgeon and spends health to do it. Jars 1 through 10 are Caviar. Jars 11 through 16 alternate. Later jars are Sturgeon Roe. If the fish is killed, every full jar becomes Caviar. Electric eels must be alive and not starving to charge the tank battery plus any battery or rechargeable machine beside the tank, blackout included. The stage rates are 1 kJ juvenile, 2 kJ adolescent, 3 kJ adult, and 3 kJ elder. The item blurb says charging begins at adolescent, while the stage list also gives juveniles 1 kJ. Both lines are kept because the source contradicts itself.

The Retired Aquafarmer comes on Tuesday if you own the Aquaculture upgrade or began as an Aquafarmer. She offers fertilized roe for all three tilapia lines, a Water Filter, and fish feed. Aquaculture II adds three large fish: an Eel or Electric Eel at day 0 for 100 cr, or a female Sturgeon at day 10 for 500 cr. The first upgrade is 400 cr, requires Water Trading, and includes one fertilized Tilapia Roe. The second is 1,200 cr, requires the first, and includes a day-0 male eel plus a day-0 female eel. Living fish stay at 0 cr in the notes. The person selling them sets the number.