An ecosystem jar is a glass container of water, plants, and small animals that is meant to look after itself. You add light. The plants grow. The animals eat and produce waste. Bacteria break the waste down. The plants use what is left. If the proportions are right, the jar can run for months with almost no help.
If the proportions are wrong, the jar tells you quickly. The water turns green. The shrimp gather at the surface. A film appears on the glass. An ecosystem jar is not a closed machine; it is a small argument between light, growth, and waste, and you are the person who sets the opening terms.
The loop, in plain terms
Four things move around a jar:
- Light enters through the glass.
- Plants use light and nutrients to grow, and release oxygen.
- Animals eat plants, algae, biofilm, or added food, and release waste.
- Bacteria and other decomposers turn waste and dead matter back into nutrients that plants can use.
The loop is never perfectly closed. Oxygen and carbon dioxide exchange through the water surface. You add food. You remove trimmings. Water evaporates. A jar is a system with a slow leak, and that leak is what makes it manageable.
The important part is the ratio. A jar with a lot of plant mass and very few animals has room to absorb waste. A jar with a lot of animals and little plant mass has no room at all. Most beginner jars fail because the second ratio is chosen first, usually because the animals are the interesting part.
What “closed” actually means
A sealed jar is not a requirement. Most successful ecosystem jars are open at the top, or covered with a lid that is not airtight. A sealed jar can work, but it is a more delicate balance: gas exchange is limited, feeding is impossible, and the system depends entirely on what was inside when it was closed.
For a first jar, leave it open. It is easier to feed, easier to top up, and much easier to rescue.
What can live in a jar
The reliable residents are small:
- Cherry shrimp (Neocaridina davidi) graze on biofilm and algae. Two or three are enough for a 2–4 litre jar.
- Ramshorn snails eat algae and dead plant matter. One is usually enough; they can reproduce.
- Nerite snails clean glass and do not breed in fresh water, but they need a mature jar with enough film to eat.
- Daphnia are tiny filter feeders that clean the water column. They are food for larger animals, so they work best in a jar without fish.
- Aquatic plants do the heavy lifting: Java moss, anubias, hornwort, marimo, water sprite.
Fish are a different proposition. A small jar can technically hold one or two tiny fish, but fish produce much more waste than shrimp, need more oxygen, and need more space to swim. A 2-litre jar with a betta is not an ecosystem; it is a waiting room. If you want fish, keep an aquarium.
The size problem
Small jars are less stable, not more. A litre of water heats up and cools down faster than ten litres. A small mistake becomes a large chemical change. The common advice is to start with 2–4 litres, which is roughly a large mason jar or an apothecary jar. A one-litre jar can work, but it is a harder first project.
The shape matters too. A jar with a wide water surface exchanges gas more easily. A tall narrow bottle has a small surface area for its volume, which makes oxygen swings more likely. Wide and shallow is more forgiving than tall and narrow.
What makes a jar stable
Three things do most of the work:
Plant mass. More plants mean more oxygen and more nutrient uptake. A jar that looks underplanted usually is. Java moss and hornwort are good because they grow quickly without much light.
Few animals. Start with two shrimp and one snail. Wait a month. If the jar is stable, add two more shrimp. The temptation to stock the jar fully on day one is the most common cause of collapse.
A consistent light period. Eight to ten hours a day is enough for most low-light plants. More light grows more algae, especially in a new jar where the plants have not filled in yet. A cheap timer removes the daily decision.
What a jar is not
A jar is not a sealed terrarium, a fish tank, or a decoration that never changes. It is a small living system with a direction. It will grow, drift, bloom, and settle. Some of that is the point. The jar is interesting because it is not entirely under your control.
If you want to try the loop before buying a jar, Pondlet’s virtual jar runs the same relationships on a screen. It is a simplification, but it is a good place to see how light, plants, animals, and waste push against each other.
Where to start
- Build a self-sustaining shrimp jar — a practical afternoon project.
- Best plants for a closed jar — what grows and what takes over.
- Does a jar need a filter? — the honest answer.
- Troubleshooting a dying jar — what to fix first.