EcoJar: Fish Tank Simulator

A sealed glass jar. Sand, water, algae, a few daphnia. Then you leave it alone and find out whether it holds.

EcoJar is an aquarium made of cellular automata. Every grain of sand falls, every bubble rises, every alga photosynthesizes, one cell at a time, across forty thousand cells. Nothing is decoration and nothing runs on a timer: if the oxygen falls, it fell because something in your jar breathed it.

THE OXYGEN BUDGET IS THE WHOLE THING
Oxygen and carbon dioxide are tracked as real quantities, produced and consumed cell by cell. Aquatic plants and algae only make oxygen where light reaches them, so the curve climbs through the day and turns over at dusk. Every animal spends it all night, whether you watch or not. A jar with too many fish and too little light does not fail dramatically. It fails quietly at four in the morning, and you find out from the chart.

THE LOOP THAT KEEPS A SEALED JAR ALIVE
Waste sinks. Bacteria break it down, and what they leave behind is nitrogen. Nitrogen drifts down until it reaches sand or a plant, feeds it, and releases a bubble on the way. Nitrogen that finds nothing returns to plain water. That is the loop a real closed ecosystem runs on, and it decides whether you get a planted tank that feeds itself or cloudy water.

WHAT YOU CAN DRAW WITH
Twelve materials, each with its own behaviour, plus Clear and Cut.
- Sand, stone, wood and glass build the jar. Sand piles into slopes and anchors anything rooted.
- Water finds its level and carries what is dissolved in it.
- Algae is the engine room: it photosynthesizes, spreads, and blooms first when there is more light and nitrogen than anything can use.
- Grass and plants are the slower producers, and what nitrogen feeds directly.
- Bacteria are the recyclers. Without them, waste accumulates.
- Daphnia and zooplankton are the food web's middle, and the difference between a clean jar and a green one.
- Fish and goldfish sit on top, and go first when the water cannot keep up.

HOW TO BUILD A JAR THAT SURVIVES
Start smaller than you want to. A nano aquarium of sand, water and algae runs a long time alone. Add one daphnia population and watch a single grazer flatten an algae bloom. Only then add a fish. Most people add the fish first, and lose that jar. A nano aquarium of sand and algae, a closed terrarium of grass and bacteria, an aquascape of plants and stone: all legitimate, all failing differently.

WHY MOST FIRST ATTEMPTS DIE
Three reasons, in order: too many animals for the algae; algae with too little light; a jar packed so tightly the gases have nowhere to buffer. None of it is scripted. The chart tells you well before the fish do.

WHAT IS NOT SIMULATED
No water temperature, because this simulation does not model it. No pH, no hardness, no ammonia number. We would rather say so than print a number that means nothing.

WHERE THIS COMES FROM
People have sealed jars since the 1960s, when David Latimer closed a bottle garden and left it shut for decades. The aquatic