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15.8. Walstad Aquarium

Photo of author

Author : David Bogert

Published :

Time To Read :
19 minutes
Diana Walstad wrote the “bible” on “low tech” planted tanks. Her book is called “The Ecology of the Planted Aquarium”. If one wants to know this topic in depth from a true expert, I heartily recommend this book (I think I’ve probably read it about five times!). She is the ONLY aquarium book author that “did it right” and based her work on what the scientific articles say about the topic rather on parroted myths and profit minded marketing hype.

I wish Ms. Walstad had written a book on fish aquariums. Her thorough and scientific research techniques guarantee it would have been worth its weight in gold. Unfortunately Ms. Walstad only wrote about the low tech planted aquarium.

Diana Walstad with one of her Aquariums
Diana Walstad with one of her Aquariums (photo used with Ms. Walstad’s permission)

The TYPICAL Walstad aquarium has the following characteristics:

Walstad
PlantsOnly easy ones
FertilizerFish food in excess
Bottom substrate1 inch (2,5cm) nitrogen poor organic soil
Capping substrate0,5 inch (1,2cm) gravel
Carbon dioxideFrom Soil and Food
LightsModerate 5 on – 4 off – 5 on – 10 off
FilterSmall
AerationNone
Water movementVery light
Number of fishLight
InoculateSoil
DifficultyEasy

Note this is ONLY a TYPICAL set up. None of these parameters are cast in stone. They can all be changed with success.

In a little more depth, a Walstad aquarium has the following TYPICAL characteristics:

  • Substrate: One-inch low nitrogen, organic, CELLULOSE RICH, packaged store bought “soil” underlayment
  • Cap: One-half inch to one-inch layer of porous gravel or large particle sand
  • Fertilization: Only fish food
  • Carbon Dioxide: No CO₂ injection, CO₂ comes from the soil
  • Lights: Moderate powered white LEDs, 4 hours on – 5 hours off – 4 hours on – 11 hours off
  • Filter: Initially “none or small filter”, now “some filtration”
  • Aeration: None
  • Water movement: Some in lower part of aquarium
  • Fish: Small number of small fish
Walstad Planted Aquarium
Walstad Planted Aquarium

A common misconception is that the “soil” in a Walstad aquarium provide plants with nutrients (nitrogen, phosphorus, potassium, iron). This is not true. The word “soil” is a misnomer. In nature “soil” is composed of largely silt, sand and clay with a small amount of organic material. The packaged “soil” from the store is largely slightly decomposed wood, bark and peat. This is the cellulose rich material used in a Walstad aquarium.

The key here is that the cellulose in Walstad aquarium will raise the carbon dioxide level in an aquarium from 2-3 ppm to 10 ppm or higher overnight. This CO₂ enrichment is key to the success of the plants in this “soiled aquarium” approach.

In nature most waterways and lakes have relatively high CO₂ levels first thing in the morning. They get this CO₂ from rotting vegetation and seepage from underground springs. So a Walstad aquarium just duplicates nature somewhat, where the “rotting vegetation” is the cellulose in the “soil” being used.

Walstad Aquarium
Walstad Aquarium

The Walstad Aquarium in Further Depth

I’ll go into some of the more complex issues of a “Walstad” planted aquarium below. This is a pretty long winded dissertation only for real nerds like the author.

Walstad Soiled Aquarium
Walstad Soiled Aquarium

Organic Soil

In general, there are two types of soil sold in big box stores: “organic” and “fortified”. “Organic” soils typically have no added fertilizers and are typically very good for Walstad aquariums. “Fortified” soils have fertilizer in them and generally need to be avoided.

What is often misunderstood about this approach is how the nitrogen level in the organic soil affects everything else. Everyone keys in on “organic” and says: “Won’t that deplete the oxygen and give me very low oxygen conditions in the soil?” The answer depends on the nitrogen. Most “organic” soils purchased packaged in the store are simply cellulose (i.e., wood, bark, and “peat”) in various stages of decomposition.

Walstad Soiled Aquarium
Walstad Soiled Aquarium

Cellulose does not oxidize readily. Wooden ships (pure cellulose) do not rapidly turn to carbon dioxide and disappear. But mix in some nitrogen into the mix, such as in a fortified soil, and bacteria and fungi can form (bacteria and fungi MUST have nitrogen to form) and somewhat rapidly turn cellulose and oxygen into carbon dioxide. At some point the oxygen levels get so low the “good” bacteria stop working. And everything then “sours” (hypoxic conditions) and starts producing all sorts of bacterial toxins.

While “sour” soil is not a problem under three inches (7,5 cm) of aquarium sand (Father Fish method), “sour” soil will be a problem under only one inch of sand. Father Fish’s grand daughter (?), one E.J.,  tried a one inch (2,5 cm) cap over a very nitrogen rich soil mix. She got a cesspool. So organic soil is fine as a one-inch (2,5 cm) underlayment IF AND ONLY IF the nitrogen level is below 0.10% (the first letter of the NPK fertilizer analysis).

Walstad Aquarium
Walstad Aquarium

Diana Walstad uses 100% Miracle Gro Organic Choice Potting Mix (MGOC). Unlike many other “potting soils”, MGOC has a clear list of ingredients and a nutritional analysis on the bag:

  • 50-55% composted bark
  • Sphagnum peat moss
  • Pasteurized poultry litter
  • “organic wetting agent” (whatever that is)
  • Analysis 0.10-0.05-0.05

This is a very good soil for the first one inch in any planted aquarium, low tech or high tech. Alternatives include peat moss and peat. Compost can be used but note the nitrogen normally found in compost can require a lot of water changes initially to remove it from the tank.

Walstad Planted Aquarium
Walstad Planted Aquarium

I set up a 55 gallon (200 liter) Walstad with Canadian sphagnum peat moss and it worked very well. I used coarse black diamond blasting media as the covering substrate. And I used a SunSun HW-603B External Canister Filter with a well baffled out flow just above the substrate. I had it for about five years. I did have to use activated carbon to remove the humic acids. I don’t like brown water.

Many say peat moss will be too acid but tests have shown all submerged soils, even those with some peat moss, tend to go to 6.0 to 7.0 pH after several weeks.

Walstad Soiled Aquarium
Walstad Soiled Aquarium

Note one can use another alternative to organic soil. One can simply buy the wood pellets used for wood stoves and put them in as the one inch soil layer. This dense product is simply pure compacted cellulose. Nitrogen, phosphorus and oxygen will slowly diffuse down from the water column into the wood layer. The wood will very slowly decompose, producing carbon dioxide very slowly.

I do recommend a phosphate addition to the wood when setting the aquarium up. This system is gone into in more depth in the following link:

15.10. Hybrid Planted Aquariums

Walstad Aquarium
Walstad Aquarium

Cap

The “cap” is the layer put on top of the soil. The cap here needs to be porous. It can be either porous gravel or porous large particle sand. It is important to have SOME exchange between the soil and the water column to keep the soil somewhat aerated. But one doesn’t want to get it too well aerated. Most vascular plant roots do best with oxygen levels at 40% to 80% full saturation.

This porosity is also important to allow the oxygen to diffuse down and slowly oxidize the cellulose in the soil. Then the resulting carbon dioxide can diffuse upward.

Walstad Aquarium
Walstad Aquarium

Fertilization

Ms. Walstad uses only fish food for her fertilizer. She adds an excess of food and allows the rotting food to provide ammonia for the plants. She did not use any biofiltration with her first set ups.  As a result of both these things Ms. Walstad had an outbreak of mycobacteriosis in her tanks.

Rotting fish food in the water column will give you a lot of heterotrophic bacteria in the water column (the term “heterotrophic” simply mean “eats normal food”) IF AND ONLY IF one doesn’t have good biofiltration with a decent filter. This in turn will give one unhealthy fish, with something like mycobacteriosis being expected.

Ms. Walstad now recommends some biofiltration in the tank. Using fish food as a fertilizer will work with a decent amount of biofiltration in the aquarium.

Walstad Soiled Aquarium
Walstad Soiled Aquarium

In leu of excess fish food one should add four drops of homemade potassium nitrate liquid fertilizer (potassium and nitrogen) twice weekly per ten gallons in the water column. And use homemade root tabs (phosphorus and iron) infrequently. The formula for making up these fertilizers can be found in this link:

15.5. Aquarium Fertilizing

Walstad Aquarium
Walstad Aquarium

Wood in a Walstad

Ms. Walstad warns against using wood in a low tech aquarium. This is because of a process called “assimilatory denitrification”. Primitive organisms like fungi, bacteria and water molds need several things to survive: carbohydrates, nitrate and phosphate.  The cellulose in the wood provides carbohydrates. The primitive organisms then pull nitrate and phosphate out of the water column and incorporate them into their bodies. This means less fertilizer for the plants. And the plants don’t do as well as they would in an aquarium without wood.

If one has wood one will need to provide more fertilizer for the plants. This is easily done if one is using chemicals for fertilization. If one is using food for fertilization one MAY have some big problems, depending on filtration. Note that wood in the aquarium is exactly the same thing as wood under the substrate and will add significant carbon dioxide to the aquarium in exactly the same way. So wood is not really “bad” for a Walstad aquarium. The wood will just require an addition of some chemical fertilizer. This necessary addition is considered “not natural” by the purists.

Walstad Aquarium
Walstad Aquarium

Carbon Dioxide (CO₂)

The Walstad aquarium does not use any carbon dioxide injection. It only uses CO₂ from the atmosphere and the natural production of CO₂ in the soil substrate. The substrate in an Walstad aquarium will add four to ten parts per million of carbon dioxide overnight. This added injection of carbon dioxide is one of the reasons the Walstad aquarium works so well.

But this also means that in five to ten years the organic cellulose in the soil layer will be depleted and the aquarium MAY stop “thriving”. Note the emphasis on MAY. Sometimes the plant roots take the place of the organics, and the roots may decompose at a similar rate.

Walstad Aquarium
Walstad Aquarium

Sanity Check

Let’s do some simple math here. One PPM per 100 gallons is 0.36 grams. One ppm per 20 gallons is 0.09 grams. Many have established that any decent planted aquarium will reduce the CO₂ levels from 10 to 4 ppm in two to three hours after a night of percolating CO₂ into the aquarium from the soil in the substrate.

This is a usage of very roughly 5 ppm per day of substrate carbon dioxide. 5 X 0.09 = 0.45 grams per day carbon dioxide. Only one third of the weight of the carbon dioxide is carbon. So this is 0.15 grams per day elemental carbon needed in a 20 gallon aquarium. A 20 gallon might have 2 pounds of dry organic soil as the first layer. The organic soil is roughly 50% by weight carbon. 2 x 0.5 x 454 = 454 grams. 454/0.15 = 3,026 days of carbon available,  3,026/365 = 8.3 years.

Now of course this is a VERY rough analysis, but it certainly validates the hypothesis that carbon dioxide from the organic substrate lasts for five to ten years and is a significant factor in the success of the Walstad aquarium.

Walstad Aquarium
Walstad Aquarium

Lights

Walstad aquariums do best with just moderate powered plain white LEDs rather than fancy high powered white-blue-crimson LEDs. Add too much light and the higher plants will be smothered by algae.

Note some put very high powered LED “shop lights” on their Walstad and proclaim that high powered lighting give the best results. LED “shop lights” lack the crimson and blue spectrum that plants utilize for photosynthesis. So if one use LED “shop lights” high power will not be detrimental. If one uses aquarium LED lights that have blue and crimson LEDs, you will typically require only 20% to 30% power.

Walstad Soiled Aquarium
Walstad Soiled Aquarium

In a tank with only water in it open to the atmosphere, carbon dioxide (CO₂) levels will settle in at about 2-3 ppm. In a typical low-tech tank with an “organic” soil substrate, the decomposition of the cellulose in the soil has taken the CO₂ level up to 10 to 12 ppm when the lights come on.

The plants in a well planted tank will then drop the CO₂ levels down to 4 ppm after three to four hours of moderate lighting. Algae like low CO₂ levels. So it is advantageous with any Walstad aquarium to do something like three hours of lighting on, three hours off, three hours on, three hours off, three hours on, nine hours off. One will get less algae. Ms. Walstad recommends a four to five hour “siesta” in the afternoon where the lights are off.

The stronger the light and the greater the amount of blue and crimson spectrum, the faster the carbon dioxide will drop. This is one of the reasons Walstad aquariums do best with just moderate powered plain white LEDs.

Walstad Aquarium
Walstad Aquarium

Filtration

Ms. Walstad initially recommended little or no filtration. And she said that the plants will remove all the ammonia just as fast as any beneficial bacteria in any filter. And she was 100% correct about the ammonia. But she has changed her viewpoint and now recommends some filtration. The reasoning has to do with heterotrophic bacteria and water clarity.

Sizable amounts of filtration through something like a small canister filled with sponge keeps the water clear because heterotrophic bacteria on the filter media remove the excess food and dissolved organics from the water column.

Walstad Soiled Aquarium
Walstad Soiled Aquarium

The trick with filtration in a Walstad is to add the filtration in such a way that the outlet of the filter is deep in the aquarium and shoots the water down or across, never up. This is to keep aeration low. Diffuse the water flow via oversized caps that reverse the flow direction and dissipate it.

The Walstad Aquarium below from a YouTube video has great plants and is very well done. But it has no filtration. Note the slightly cloudy water (in the video on YouTube the cloudiness is much more apparent). This is what to expect when one doesn’t use filtration.

There are a whole series of small DIY filter designs that will work very well with this type aquarium (low flow deep in the tank) and can be seen at the following article:

8.7.8. Small DIY Filters

Walstad Aquarium that has no filter
Walstad Aquarium that has no filter

Aeration

Aeration is the enemy of a Walstad aquarium. No aeration allows the carbon dioxide levels to get higher than 10 ppm. Greater than 10 ppm carbon dioxide gives the best higher plant growth. This, in turn, creates a problem having to do with fish stocking.

Walstad Aquarium
Walstad Aquarium

Fish

Walstad aquariums need small stockings of small fish. Most folks understand the linear relationship of the number of fish with relationship to stocking. I.e. four two inch (5 cm) fish consume four times the oxygen of one two inch (5 cm) fish. But most people fail to comprehend the size relationship, which is what is called a cubed relationship. A two inch (5 cm) platy consumes EIGHT times the oxygen of a one inch (2.5 cm) tetra.

Let us say someone has a ten-gallon (37.8 liter) Walstad aquarium and puts twenty full grown platies into the tank. And the tank has no aeration. The next morning the hobbyist will awake to twenty dead platies. Note that if one is taking one inch (2.5 cm) tetras one will need 160 tetras to do the same thing,

Walstad Soiled Aquarium
Walstad Soiled Aquarium

Fish need oxygen. They need a lot of it.  And while plants supply oxygen during the times they have light on their leaves, at night plants don’t do that, they even use a miniscule amount of oxygen at night. This aeration problem means that Walstad aquariums require low stocking of small fish. More about what constitutes “light stocking” go to this link:

14. Stocking

Walstad Aquarium
Walstad Aquarium

Water Movement

Plants do best with some movement of the water over their leaves. This movement brings the plants the carbon dioxide and nutrients they need. But this movement needs to be very light. “Choppy waves” at the surface need to be avoided as this will aerate the tank too much. In small tanks this can be a real challenge as even a 5-watt bladed circulation pump is too much current. And air stones need to be avoided.

So for small Walstad aquariums I take small 3-watt fountain pumps and direct the flow into a two inch pipe filled with pot scrubbers to do biofiltration. The pipe is easily hidden under the substrate at the back of the tank. The output from the pipe goes from a half inch (12 mm) tube butting into a large one inch (25 mm) PVC cap painted black. The cap dissipates the flow but still allows a little water movement. For larger aquariums the flow can be from a 5-watt powerhead, again, directed into a larger cap to dissipate the flow.

Walstad Aquarium
Walstad Aquarium

Inoculate

If one uses a sterile soil like peat moss and some packaged soils in a Walstad planted aquarium there can be an issue. The planted tank is a miniature ecosystem. And ecosystems do best with a diversity of creatures in them. The more species of bacteria, fungi, protozoans and metazoans one has in a planted tank the healthier the plants will be. Will a few species be pathogenic? Yes. But they will be outnumbered a thousand to one by the beneficial species.

Both Father Fish and the author go out and get pond mud to inoculate a new tank. Adding one teaspoon dark pond mud to the soil of any planted tank provides something called “biodiversity”. This biodiversity is very important to the health of the plants in a Walstad aquarium. We go into this in more depth in this link:

2.14. The Mature Aquarium

Walstad Aquarium
Walstad Aquarium

Ammonia Removal

There are largely three separate ways in which ammonia can be removed from an aquarium:

  • Plants can absorb it. Plants then use high energy carbohydrates they make using light as an energy source to convert the ammonia into proteins in their bodies. Note algae are plants.
  • “Beneficial” or autotrophic nitrifying bacteria convert ammonia to nitrate. They use the very low energy produced by oxidizing ammonia to create proteins they use in their bodies. Plants can and will outcompete “beneficial” bacteria for ammonia.
  • Some species of facultative heterotrophic bacteria consume both ammonia and free carbohydrates in the water column (the term “heterotrophic” simply means “”eats normal food”). They use the high energy of the carbohydrates together with the ammonia to create proteins they use in their bodies. Heterotrophic bacteria can and will outcompete both plants and autotrophic bacteria in the absorption of ammonia.
Walstad Soiled Aquarium
Walstad Soiled Aquarium

If one adds 8 grams of sugar and 1 gram of ammonium salt to an established unplanted aquarium the ammonia will be gone in two hours. And the aquarium will become cloudy. This is due to the heterotrophic bacteria. Heterotrophic bacteria can out compete plants and autotrophic bacteria for ammonia uptake IF the bacteria have a carbohydrate source.

Most fish food has very roughly 40% proteins in it and 40% carbohydrates in it. So when one adds fish food as fertilizer to a planted aquarium, a goodly portion of the ammonia produced will be consumed by heterotrophic bacteria in the water column. They will use the carbohydrates as their energy source. These heterotrophic bacteria in the water column remove some of the ammonia the plants need and they create unhealthy conditions for the fish.

Walstad Aquarium
Walstad Aquarium

Nitrifying bacteria will not be significant in a planted aquarium simply because nitrifying bacteria use the oxidation of ammonia as their energy source. This very low source of energy cannot compete with the higher energy conversion by plants and heterotrophic bacteria.

What is revealing is that Ms. Walstad had nitrates build up in her aquariums and had to do a water change every four months or so. Ms. Walstad does not actively harvest the plants in her aquariums. Had she harvested the plants she would not have needed to do any water changes.

And let me emphasis that anaerobic reduction of nitrate to nitrogen gas simply does not occur in ANY aquarium, even deep sand bed aquariums. Testing has repeatedly shown this simply is a myth. For more on this go to this link:

14.2.4. The Anaerobic Myth

Walstad Aquarium
Walstad Aquarium

Depleted Substrate

Many who do the Walstad method find that the plants start dying after five to ten years. Many ascribe this to a “depletion of nutrients” in the soil. Since the soil in a Walstad aquarium doesn’t provide nutrient (nitrogen, phosphorus, potassium, iron) this idea is incorrect. The reduction in growth is due to the depletion of the cellulose in the soil and the resulting lack of CO₂.

Walstad Soiled Aquarium
Walstad Soiled Aquarium

Fertilizer in the Substrate

The Walstad approach emphasizes no chemical fertilizers and a completely “natural approach”. But root tab fertilization of the substrate helps a great deal with Walstad aquariums.

But note that this fertilization has to be a very light handed approach. Use small root tabs sparingly every year. And one should sprinkle some (like half a teaspoon per five pounds soil) calcium phosphate (any form of calcium phosphate purchased from the internet) and some Epsom salts into the soil before laying it down into a Walstad aquarium.

Go to the following link for more on how to do this:

15.5. Aquarium Plant Fertilizing

Walstad Aquarium
Walstad Aquarium

Planted Aquariums in Depth

The following sections will tell you how to make a planted aquarium:

15. Planted Aquariums

15.1. Planted Aquariums in Depth

15.2. Fish for Planted Tank

15.3. Fish Limitations

15.4. Types of Planted Aquariums

15.5. Fertilizing

15.6. Carbon Dioxide

15.7. Substrates for Planted Aquariums

15.10. Hybrid Planted Aquariums

15.11. Many Fish Many Plants

Walstad Soiled Aquarium
Walstad Soiled Aquarium