Walstad Bowl Planner

How to use the Walstad Bowl Planner

Built from a working Walstad bowl — not theory. Here is what the planner is doing, why each layer matters, and where people go wrong.

What the Walstad method actually is

Diana Walstad's method uses organic potting soil under a sand cap as the substrate, with heavy planting and minimal filtration. The soil feeds the plants; the plants filter the water. Done right, a Walstad bowl can run for years with no filter, no CO2, no fertiliser, and only occasional top-ups. Done wrong, it is a murky, smelly mess that kills everything in it. The difference is almost entirely in the substrate layers — which is what this planner calculates.

What the planner calculates

You enter your bowl dimensions and the planner works out four things:

Soil volume — enough organic potting soil to give plants 2.5–4 cm of root zone. Too little and the plants starve; too much and you risk anaerobic pockets that release hydrogen sulphide.

Sand cap volume — at least 2 cm of play sand over the soil to seal it. The cap is what stops soil leaking into your water column. Skip it or skimp on it and you will have brown water for weeks.

Actual water volume — your bowl's total volume minus substrate displacement. This is the number you need for stocking and dosing, not the nominal bowl size.

Mineral layer (optional) — crushed coral or aragonite under the soil for soft-water setups that need buffering. Skip this if your tap water is already hard.

The 12-step build checklist

When you run the planner, you get a checklist that walks you through the entire build — from rinsing the sand to adding livestock at day 7. Do the steps in order. The order matters: if you flood before planting, you will never get the plants rooted properly. If you add fish before day 7, the bowl has not settled and you will lose them.

Common mistakes that kill Walstad bowls

Using the wrong soil. The soil must be organic potting soil with no perlite, no moisture crystals, no fertiliser pellets, no fungicide. Perlite floats. Moisture crystals turn into slime. Fertiliser pellets cause ammonia spikes. Read the bag. If it says "slow release" anywhere, put it back.

Skipping the sand cap. I have seen people try to save a few dollars by skipping the cap. Every single time, the water turns brown, the soil goes anaerobic, and the bowl has to be torn down and restarted. The cap is not optional.

Planting too lightly. A Walstad bowl needs 70%+ plant coverage at planting, not the 30% that looks "natural". The plants are your filter. Under-plant and you get algae blooms within the first month because there is nothing to outcompete it.

Doing full water changes. Walstad bowls run on stability. A 20% weekly change is correct. A 100% change resets the bacterial balance and you start over. The only time you do a full change is if the bowl has crashed — and if it has crashed, you have a bigger problem than water changes.

Frequently asked questions

Do I really not need a filter?

For bowls under 15 litres with shrimp or snails only, no. For bowls with fish, you need at least an air-driven sponge filter — the plants handle nitrogen, but surface agitation matters for oxygen exchange. The planner will tell you if your chosen livestock needs filtration.

Can I use dirt from my garden instead of potting soil?

You can, but I would not recommend it for your first Walstad bowl. Garden soil composition is unknown — it may be too rich in organics (causes ammonia spikes) or too clay-heavy (compacts and goes anaerobic). Organic potting soil is predictable. Once you have done two or three successful bowls, experiment with garden soil if you want.

How long until I can add fish?

Seven days minimum, and only if ammonia and nitrite both read zero on a test kit. Walstad bowls do a "silent cycle" — the plants absorb ammonia directly, so you may never see a nitrite spike. Test anyway. If ammonia is above 0.25 ppm at day 7, wait another week.

My water is cloudy on day 3. Is the bowl broken?

No — this is normal. The first bacterial bloom usually clears within 7–10 days. Do not do a water change. Do not add chemicals. Leave it alone and it will resolve. If it is still cloudy at day 14, your sand cap was too thin and soil is leaking — do a 30% change and add another centimetre of sand.

Can I put a betta in a Walstad bowl?

Yes, in a bowl of 15 litres or larger with a small heater (bettas need 25–27°C). The planner will recommend betta-safe plants and tank mates. Do not put a betta in anything under 15 litres — they need horizontal swimming room, not just water volume.