How It Works

Math Questions

Choose a grade from K to 9 and answer math questions to earn coins. Correct answers give coin rewards, and wrong answers move you to the next question.

Coins

Coins buy fish, food, tanks, passes, and koi room items. Fish do not add coin bonuses; rewards come from the math questions.

Aquariums And Game Saves

Each aquarium is its own game save. New Aquarium makes another aquarium without deleting your older ones. Save lets you check one or more aquariums to include, then give that saved group a save name and password with at least 8 characters. If that name and password already have a save, the game asks before replacing it. Add Saved Aquariums uses the same name and password to add the saved group back into your aquarium list without deleting the aquariums already there.

Fish, Food, And Tanks

Fish start small and grow over time. Bigger fish need more tank space. Food is eaten slowly. If the tank runs out of food, emergency feeding costs coins and can create debt if you cannot pay. New fish get numbered names like minnow1, minnow2, or sankekoi1, and Rename Fish lets you change any fish name later.

Tank Space

Small, medium, big, large, and koi tanks hold different amounts of fish space. Better or larger fish take up more room. If fish no longer fit, you may need a bigger tank or release fish.

Koi Room

The koi room unlocks later with a pass. Koi are separate from regular fish and need koi tanks. Koi tanks are bought in the koi room, and the largest koi tank gives unlimited koi space. Koi breeding also happens in the koi room: choose two adult or giant koi, pay the breeding cost, and a baby koi hatches. You can also use Random Mix From Grown Koi to let the game choose two grown koi and a random parent mix for you. Bred koi hatch with mixed parent patterns. You can choose the parent mix, such as 65% one parent and 35% the other, and bred koi can carry several koi lines forward. They start small, save with your aquarium, and grow into more space. Premium Koi Growth Food is a pricey koi-only item that speeds up koi growth, but bigger koi may need bigger koi tanks. The live koi market lets a breeder choose a bred koi and set its coin price. Other players can buy the listing to receive a one-time code, then redeem that code to add the koi. The seller can unlist an unsold koi to return it to the aquarium, and completed sales add the coins back to the seller's aquarium.

Pause And Music

Pause stops earning and fish care changes until you resume. Music can be turned on or off from the top of the game.

Offline Save Protection

Device aquariums work without an internet connection. Mathematical Trout keeps a sealed last-trusted copy in the browser's private offline storage. If the ordinary save is directly changed or damaged, the game restores the trusted copy and shows a warning. Clearing all browser data still removes device-only aquariums.

Parent Center

A grown-up can create a 4 to 8 digit parent PIN from Set Up Parent PIN at the top of the game. A highlighted Parent Setup or Parent Login button also stays in the sticky game navigation while the page is scrolled. The lock protects aquarium deletion, online saves and loads, public sharing, koi market selling, privacy choices, and reset controls. Math questions, fish care, decorating, breeding, and purchases made with game coins remain available. The Parent Center also shows a learning summary for each aquarium on this device, and a parent can choose its grade level there. Creating a PIN locks the Parent Center immediately so it can be tested with Enter Parent PIN. It locks again on refresh, when Lock Now is pressed, or after 10 minutes. The PIN cannot be displayed or recovered; clearing site data removes the lock but also removes device-only aquariums.

Leaderboard

Each normal aquarium has its own Publish button. A published aquarium shows its aquarium name, a nickname for who made it, coins, fish count, koi count, and score points. Use a nickname, never a full name or contact details. The score uses coin points, regular fish rank points, koi rarity points, tank points, and the current streak. Update Board refreshes its score, Edit By changes the nickname, and Remove Board takes that aquarium off again. The local test room cannot publish to the aquarium leaderboard because it uses testing coins.

Bred Koi Showcase

A koi you bred can be published separately in Bred Koi Likes. Choose exactly which bred koi to publish or unpublish. Each browser can like a koi once and can press the same button again to remove that like. Renaming a published koi also updates its showcase name.