How to Set Up a Walstad Tank: Soil, Sand, and No Filter

The short version

• A Walstad tank is a planted aquarium built over a thin layer of ordinary soil, capped with sand, that runs with no filter and no CO2.
• The soil feeds the plants, the plants and substrate bacteria do the filtering, and the whole thing costs about the price of a bag of dirt.
• Expect the water to cloud for the first 7 to 10 days while the bacteria catch up, then clear on its own. That cloudy week is the only hard part.
• Below: the layers, the fill, the first month, and what to stock in a 10-gallon once it is stable.

A dirted tank looks like witchcraft the first time you see one: a jar of plants and fish with no filter humming away, clear as glass, running for years on a bag of topsoil. It is not witchcraft. It is Diana Walstad's Ecology of the Planted Aquarium put into a tank, and the biology is straightforward once you build it in the right order.

The soil under the sand is the whole trick. It feeds the plant roots directly, and the plants grow fast enough to pull ammonia and nitrate out of the water before it can build up. The substrate and plant surfaces grow the same bacteria a filter would, so you do not need the filter. Here is how to set one up.

What a Walstad tank actually is

A Walstad tank is a low-tech planted aquarium with a nutrient layer of plain soil under an inert cap, heavily planted from day one, lit for 6 to 10 hours, and left to balance without a filter or injected CO2. It is named for Diana Walstad, whose 1999 book made the case that a tank can run on soil, plants, and light alone.

The definition matters because two things get called Walstad that are not. A bare-bottom tank with a sponge filter is low-tech, but it is not dirted. A high-tech aquascape with pressurized CO2 and aquasoil is planted, but it is the opposite of Walstad. The method is specifically dirt plus heavy planting plus low technology, and the balance comes from biology, not equipment.

The layers, bottom to top

Build the substrate in two layers in a 10-gallon tank or larger. Smaller than 10 gallons and the water chemistry swings too fast for a beginner to hold steady.

1. Soil, 1 to 1.5 inches. Plain organic topsoil, the cheapest bag with no added fertilizer, no manure, and no perlite. Those additives spike ammonia and float. Mist the soil damp and press it flat.
2. Sand cap, 1 inch. An inert sand over the soil, the same depth across the whole floor. The cap keeps the dirt down and stops it clouding the water every time a fish digs. One inch is the number: thinner lets soil leak up, thicker goes anaerobic.

That is the entire substrate. No gravel, no root tabs, no base fertilizer. The soil is the fertilizer, and it lasts years before it runs down.

Planting and the first fill

Plant heavily before you add much water, while the substrate is just damp. Heavy planting from day one is not optional in this method: the plants are the filter, so a sparse tank is an unfiltered tank with nothing doing the work.

Weight the plant list toward fast growers for the first months. Fast plants pull ammonia hardest during the fragile early weeks, which is exactly when you need it. A good beginner mix for a 10-gallon is a stem plant or two, a floating plant for shade, a slow rooted plant for the midground, and an epiphyte on a piece of wood.

Fill slowly to avoid churning the sand into the soil: lay a plate or a plastic bag on the substrate and pour onto that, or run water down the glass. Fill to the top, then turn on the light for 6 to 8 hours a day on a timer.

The cloudy first week, and why you wait

For the first 7 to 10 days the water will cloud, sometimes milky, sometimes a green tint, sometimes tea-brown from tannins in the soil. This is normal, and it is the single most common reason beginners tear a dirted tank down before it works.

The milkiness is a bacterial bloom: the colony that processes ammonia is exploding to match the food the new soil is releasing. It clears itself once that colony catches up, usually inside two weeks. Do not add a filter, do not do daily water changes, and do not add animals yet. A large water change here just resets the bloom and drags it out.

What you can do: keep the light to 6 hours a day for the first two weeks so algae does not get a foothold while the plants root in, and top off evaporation with dechlorinated water. Test the water so you are reading the tank, not guessing.

Cycling and when it is safe to stock

A Walstad tank still cycles: ammonia from the soil converts to nitrite, then to nitrate that the plants consume. You want to see ammonia and nitrite both read zero on a liquid test before any animal goes in, which usually lands somewhere between week 3 and week 6.

Plants shorten the cycle because they eat ammonia directly, and a heavily planted dirted tank often shows very low ammonia the whole way through. That is a good sign, not a reason to skip testing. Wait for two consecutive zero readings a few days apart, then stock lightly.

The order of stocking matters. Start with the cleanup crew and the hardiest animals, give the tank a week to absorb that bioload, then add the centerpiece. Light and slow beats heavy and fast every time in a filterless system.

What to stock in a stable Walstad 10-gallon

Stock a filterless tank at roughly half the density you would a filtered one. Without a filter, the plants and substrate are your only buffer against a bioload spike, so understocking is the safety margin.

A calm, self-sustaining 10-gallon can hold a small school of a cool-water fish, a colony of dwarf shrimp, and a few snails that work the substrate. The snails and shrimp are not decoration here: they are part of the cleanup crew that keeps the closed loop closed, grazing biofilm and turning leftover food and detritus back into plant nutrients.

"Self-sustaining" does not mean you never touch it. You still top off evaporation weekly, feed lightly, prune the fast plants when they reach the surface, and do the occasional water change if nitrate climbs. The biology handles the filtration; the keeper still keeps.

Frequently asked questions

Do you really never need a filter in a Walstad tank?

In a balanced, heavily planted dirted tank, no: the plants and the bacteria on the substrate do the filtration a filter would. Many keepers still add a gentle sponge filter for water movement and insurance, and there is nothing wrong with that. It just is not required once the tank is planted heavily and stocked lightly.

How long until a Walstad tank is stable?

Plan on 3 to 6 weeks. The visible cloudiness clears in about 7 to 10 days, but the tank is not cycled and ready for animals until ammonia and nitrite both read zero on a test, which takes several weeks. Rushing an animal in during the bloom is the fastest way to lose it.

What soil should you use for a dirted tank?

Plain organic topsoil or a basic organic potting soil with no added synthetic fertilizer, no manure, and no perlite or wetting agents. The cheapest plain bag at a garden center is usually the right one. Cap it with an inch of inert sand.

Can you keep fish in a filterless dirted tank?

Yes, as long as you stock lightly, wait for a full cycle, and choose fish suited to calm, unfiltered water. A cool-water schooling fish plus a shrimp-and-snail cleanup crew is a classic, low-bioload starting point for a 10-gallon.

Once the tank is stable, the choices that keep it that way are all compatibility questions: which fish suit your water, how many the volume can hold, and which plants will actually grow under your light. Run your setup through the build planner for a stocked, balanced starting point, or read the rest of the planted-aquarium guides for the next build.