Water & oxygen

Dissolved oxygen

Oxygen is measured as dissolved oxygen (DO) in milligrams per litre, mg/L, the same unit as a real pond test kit. How much the water can hold depends on its temperature: cold water holds more, warm water less. Full water is called saturated, and no amount of air will push it higher.

This is why summer is the dangerous season. The water holds the least oxygen just when koi are most active and breathe the most. Winter is the opposite: cold water holds plenty and the koi barely use any.

What the numbers mean

• Growth is at full speed from 6 mg/L, slows steadily below that, and bottoms out at about a third of normal at 4 and under.
• Gasping at the surface is called piping. You’ll see the koi bunch up by the bubbles and tiny rings where their mouths break the surface. It’s a warning, not yet a death sentence.
• A pond tab over the water shows a pulsing O₂ badge when a pond with fish drops below 4.

How much koi breathe

Every koi uses oxygen all the time, and size matters most. A koi’s weight grows with the cube of its length and its oxygen use with about weight^0.8, so use grows with length^2.4. A 60 cm koi uses about five times what a 30 cm koi does.

use per koi = (length ÷ 40 cm)^2.4 × warmth

Warmth follows the water: koi are cold-blooded, so a koi at 6°C breathes about a quarter as hard as it does at 25°C. Fry hardly count at first; a pond of tosai in autumn still uses less than one big adult.

Pond size spreads the load. The same koi takes less out of a bigger body of water. Each pond’s volume grows with its capacity: a show pond has 1 unit of water per fish space, the breeding pond 3 (it’s deep), and the mud pond a quarter (it’s sized for fry). Upgrading a pond’s capacity gives every fish in it more water to breathe.

Where oxygen comes from

Both kinds of supply push towards saturated, so they work hardest when the water is low and ease off as it fills. That means the oxygen always settles at a level where what goes in matches what the koi take out:

settles at = saturated − koi breathing ÷ (1 + air stones)

The Farm tab shows that level as a dashed line on each pond’s oxygen bar. If it’s in the red, the pond will get there whatever you do, unless you add air, thin the stock or wait for cooler weather.

• A water change adds about 1.5 mg/L at once, because fresh water carries oxygen. Handy in an emergency, but it wears off.
• Hot, still nights (a summer event) knock 2.5 mg/L off a pond with no air stones and stress the fish. With a stone running it’s only 1.2, and the fish are fine.

Air stones

An air stone is a porous stone on the end of an air line. A pump on the bank pushes air down the line and the stone breaks it into a stream of fine bubbles. In the game you’ll see the line running in from the right-hand edge and a boil of foam and rings on the surface above each stone.

• Buy stones in the Upgrades tab. Each pond takes up to 4 stones on its pump.
• A new stone drops in at the best free spot. Drag it to move it; while it’s lifted out of the water it stops bubbling.
• Sell a stone from the Care tab for half what it cost.
• Stones are free to run for now. Running costs arrive with energy in a later update.
• Older saves with the old aerator upgrade get a ball air stone in its place.

Placing stones

Where a stone sits changes how much it does. Each stone shows its efficiency as you drag it, and the Care tab lists every stone with its efficiency and why.

• Spread them out. Two stones close together share the same water, so each does less. Right on top of each other, two stones do about the work of one.
• Keep them off the walls. A stone against the edge only stirs half a circle of water and loses up to 35% of its output. A tenth of the way in is enough.
• The centre is fine for one stone. With several, think corners of a square, well in from the edges.

How many stones?

Try a pond. This uses the game’s own formulas.

Assumes every stone is well placed. Stones that share water or sit against a wall do less.

Oxygen and water quality

Fish waste and uneaten food are broken down by bacteria living in the pond and the filter. Those bacteria need oxygen too. In the game:

• The bacteria work at full speed from 7 mg/L, slow down as oxygen falls, and are down to 10% at 2 mg/L. The Farm tab shows this as Bacteria at work.
• Everything that cleans the water each day (the pond itself, filters and plants) is scaled by the bacteria: at full speed you get it all; with starved bacteria only a little over half.
• So low oxygen makes water quality fall faster, and poor water makes fish sick. A stone in a crowded pond pays for itself twice: more oxygen, and cleaner water.
• Oxygen doesn’t stop the waste being made. A heavily stocked pond still dirties quickly even with good air, which is what filters, plants and water changes are for.

Reading the Farm tab

The Farm tab has a card for every pond:

• Dissolved oxygen with a coloured bar: red below 2, dark red to 4, amber to 6, green above. The white marker is now; the dashed line is where it will settle; the faint line is saturated for today’s temperature.
• Oxygen per day: koi breathing (red, taken out), surface exchange and air stones (blue, put back), and the change per day.
• Water quality, Bacteria at work, the Feeder level, Food use per day and Stocking.
• The pond tabs over the water show each pond’s fish count and its feeder level, so you can see a hungry pond without opening it.

Real ponds

• Koi keepers aim to keep dissolved oxygen above about 6 mg/L. Koi tolerate less for a while, but they stop feeding well, and below about 2–3 mg/L they’re in real trouble.
• The worst time is just before dawn on a hot summer night. Pond plants and algae use oxygen in the dark, and the warm water can’t hold much.
• Air stones mostly work by moving water, not by the bubbles themselves. The rising column of bubbles drags low-oxygen water from the bottom up to the surface, where it picks up air. That’s why the game counts the surface boil, and why stones in a corner do less.
• Big koi farms in Japan run paddle-wheel aerators or air lines in every mud pond through the summer, and stock lightly so the fish grow fast.