The Walstad Jar: A No-Filter Ecosystem on a Desk

The short version

• A Walstad jar is a 1 to 3 gallon glass jar built on the same idea as a dirted tank: a thin soil layer, a sand cap, heavy planting, light, and no filter or heater.
• It holds invertebrates only. Cherry shrimp and snails, never fish, because a jar under 2 gallons swings parameters too fast for anything with a real bioload.
• Build it, plant it heavily, cycle it for 3 to 6 weeks, then add a colony of shrimp. The plants and soil do the filtering.
• It is the smallest honest self-sustaining build and the least forgiving, because a small water volume gives you almost no margin for error.

A jar of plants and shrimp sitting on a desk with no filter, no heater, and no cords is the purest version of the whole idea: soil, plants, and light holding a tiny closed loop steady. It is also the version with the least room for mistakes, because everything that makes a big tank stable, water volume most of all, is in short supply in a jar.

Done right, a Walstad jar is a genuine little system that runs for years on topping off and light feeding. Done wrong, it is a cloudy jar that crashes a colony in a week. The difference is almost entirely in respecting what a small volume can and cannot do.

What a Walstad jar is, and what it can hold

A Walstad jar is a wide-mouth glass jar or vase of 1 to 3 gallons, set up like a dirted tank in miniature: a thin layer of plain organic topsoil, an inch of inert sand over it, planted heavily, lit by a small clip light, and run with no filter and no heater. The plants and the soil bacteria handle the filtration, exactly as they do in a 10-gallon.

What it can hold is the hard limit. A jar is an invertebrate home: a colony of cherry shrimp (Neocaridina davidi), or a few snails, and nothing with a real bioload. Fish do not belong in a jar, even nano fish, because parameters swing hard under 2 gallons and a jar has no filter to buffer them. Cherry shrimp work precisely because their bioload is very low and they stay under 1.2 inches full grown.

Building the jar, step by step

Build it in the same order as any dirted tank, just smaller and more carefully.

1. Soil, about 0.75 to 1 inch. A thin layer of plain organic topsoil with no added fertilizer, manure, or perlite. In a jar you use less than an inch, because a deep layer in a small volume goes anaerobic fast.
2. Sand cap, 1 inch. An inert sand over the soil, level across the bottom, to hold the dirt down and stop it clouding the water every time something digs.
3. Plant heavily while damp. Push in java moss (Taxiphyllum barbieri), a stem or two of hornwort, and a marimo moss ball. Heavy planting is the filter, so a sparse jar is an unfiltered jar with nothing doing the work.
4. Fill slowly. Lay a spoon or a small dish on the sand and pour onto that so you do not churn the soil up into the water.
5. Light it 6 to 8 hours. A nano clip light on a timer is plenty. Hold it to 6 hours for the first two weeks so algae does not get a foothold.

Why a jar is invertebrate-only

The whole reason a jar is shrimp-and-snail territory comes down to water volume. In 1.5 gallons, a fish's waste, a missed water top-off, or a warm afternoon moves ammonia, temperature, and pH far faster than the same event would in a 10-gallon. There is no filter and very little water to soak up the swing.

Cherry shrimp fit because they add almost nothing to that balance: their bioload is very low, they graze biofilm and leftover food all day, and they hold happily at room temperature between 65 and 78 F with no heater. Snails that work the substrate suit a jar for the same reason. A Malaysian trumpet snail (Melanoides tuberculata) burrows and aerates the sand, and a ramshorn snail grazes algae, and neither pushes the bioload where a jar cannot follow.

Stocking the jar

Wait for a full cycle before a single shrimp goes in. A jar cycles like any dirted tank, ammonia to nitrite to nitrate, and it is not ready until ammonia and nitrite both read zero on a liquid test, usually somewhere in the 3 to 6 week window. The bloom-and-clear cloudy phase in the first 7 to 10 days is not the cycle finishing; testing is.

Once it is cycled, add shrimp in one modest batch. Cherry shrimp are a colony animal and do best starting around 10, which a 2 to 3 gallon jar can carry because their bioload is so low. Add snails if you want a cleanup partner, and stop there. The point of a jar is a stable low bioload, so resist stacking it. Shrimp need minerals in the water to molt (a general hardness of 4 to 14 dGH), and copper, which turns up in some plant fertilizers, is lethal to them, so keep it out of the jar.

The honest part: a small jar swings fast

A jar gives you almost no margin, and that is the trade for its size. Evaporation alone concentrates everything left behind, so a jar that goes a week without a top-off can climb in hardness and temperature enough to stress a colony. You top off with dechlorinated water little and often, not in one big weekly dump that shocks the parameters the other direction.

Temperature is the other trap. With no heater, the jar rides the room, so a jar on a sunny sill can cook past a shrimp's comfort on a hot afternoon and chill at night. Keep it out of direct sun and away from vents. And be honest with yourself about the failure mode: a jar is more fragile than a 10-gallon, not less, and the "self-sustaining" part refers to the filtration, not to leaving it alone. You still test, top off, feed a pinch, and watch. If you want the easiest possible first build, a 10-gallon is more forgiving than a jar in every way but shelf space.

Frequently asked questions

Can you keep fish in a Walstad jar?

No. A jar of 1 to 3 gallons swings parameters too fast for fish, even nano species, and it has no filter to buffer them. A jar is for invertebrates: cherry shrimp and snails, whose bioload is low enough for the volume. Fish need a cycled, filtered, or heavily planted tank of at least 5 gallons, and most want more.

How many shrimp can a Walstad jar hold?

A 2 to 3 gallon jar comfortably starts a colony of around 10 cherry shrimp, and the colony self-regulates from there to what the food and biofilm support. Their bioload is very low, which is exactly why a jar can carry them where it could carry no fish.

Does a Walstad jar need a filter or heater?

No, by design. The plants and soil bacteria do the filtration, and shrimp and snails hold at room temperature (65 to 78 F for cherry shrimp) with no heater. What it does need is heavy planting, a real cycle, and steady room conditions, because it has no equipment to cover for a swing.

How long before a Walstad jar is ready for shrimp?

Plan on 3 to 6 weeks. The water clears in the first 7 to 10 days, but "clear" is not "cycled." Wait until ammonia and nitrite both read zero on a test a few days apart, then add the shrimp in one batch.

A jar is the smallest place this idea works, which makes it a sharp teacher of the fundamentals. Plan a stocked, balanced jar on the build planner, check a shrimp's or snail's range in the compatibility database, and read the cheapest way to start a planted tank if budget is the driver, the common Walstad mistakes before you make them, or floating plants for low-tech tanks for the nutrient sponges a jar loves. More builds sit in the planted-aquarium guides.