20 Gallon Walstad Method Natural Planted Tank

Published June 30, 2016

My wife and I have been running our 20 gallon Walstad method planted tank since March 2014. It started off as an experiment and a year later has been doing great. This is a photo of our Walstad tank.

20 gallon Walstad tank

20 gallon Walstad tank (el natural)

The last update was in July 30, 2016. Jump to Updates.


For those who are new to this, Walstad method is a natural planted tank (NPT) also known as El Natural tank, and was created by Diana Walstad, a well-known ecologist. This method describes a way of setting up an aquarium where plants and fish balance each other's existence and needs. It's almost self-sustaining. The full explanation of this process is in Diana Walstad's book "Ecology of the Planted Aquarium".


This is our setup:

  • Aquarium: 20 gallon glass tank (Petsmart)
  • Substrate: 1-1.5 inches of Lowes top soil capped with 1-1.5" of Petco black sand
  • Fertilizer: None
  • Iron supplement: Amaco Mexican pottery clay. You can also buy it on Amazon if you don't live near a store.

  • Lights: Two 13W CFL bulbs. They are 5000K daylight bulbs and fit in the default fixtures. The tank came with two T10 lights, which are too dim for any kind of plants, so we replaced them with CFL bulbs.
  • Air Stones: Two small airstones for backup oxygen and minimal water movement.
  • Filter: No filter. The plants act as a biological filter.
  • Heater: No heater. The aquarium stays at 72-74 Fahrenheit (room temperature).
  • Water Changes: We did three 10% water changes in the last one year. And top-offs every few weeks. For the first 6 months, we didn't do any water changes.
  • Plants: Several low light stem and floater plants.
  • Invertebrate: Nerite snail, several Malaysian Trumpet Snails and Ramshorns.
  • Fish: Danios and Guppies.


If you've already read Diana Walstad's book, you'll understand this process better.

Place the empty 20 gallon tank on a sturdy table. When filled, it will be 200 pounds or heavier.

Prepare the soil

Throw the top soil into a Rubber Maid container and manually remove the twigs and stones. Wash it, pour out the excess water and leave it outside overnight. Next morning, pour out the water on top. You'll get rid of the floating twigs and debris. Repeat this wash-rinse step twice over the next two days. Some people mineralize it by following a wash-rinse-dry cycle. We couldn't do it because it was in the middle of winter and drying it outside was not an option.

NOTE: Lowe's brand (gardenpro) topsoil has way too many twigs and stones compared to Scotts Premium topsoil. For our second tank (10 gallon), we used Scotts Premium topsoil from Home Depot

Wash the sand

Wash the sand thoroughly. Some brands of sand can be dirty or oily, so it's important to wash them beforehand.

NOTE: We used Petco's black sand for our 20 gallon. I won't really recommend this for a Walstad tank because it is too fine and can cause anaerobic pockets that will kill your soil. For our second tank, we used Black Diamond blasting sand. There's a lot of washing to do with the blasting sand, but it's worth it. Instead of Black Diamond blasting sand, pool filter sand is good too. Just make sure you get medium grade sand and not fine sand.

Amaco Mexican Pottery Red Clay

This is useful because it contains iron that's very beneficial for the plants, especially red plants like ludwigia repens. I got mine from Hobby Lobby. It is sold commercially as Amaco Mexican Pottery Red Clay. A little clay goes a long way and can be used for several tanks.

Roll the clay into several small balls about 0.5-1" in diameter. Dry them for a few hours. Make enough that will spread apart by 1.5" to 2".

Keep the plants ready

If you haven't already got the plants, buy them either from your local LFS or other aquarium owners. Also look out for aquarium owners giving out free plants.

This is our plant list:

  • Floating plants: Amazon Frogbit
  • Stem plants: Anacharis densa, Anacharis narrowleaf, Ludwigia repens, Moneywort, Rotala Indica, Rotala Rotundafolia
  • Other rooting plants: Amazon sword, Walmart Aponogeton, Vallisneria Spiralis Leopard
  • Rhizome plants: Anubias Coffeefolia, Java Windelov Fern
  • Moss: Java moss, Marmimo moss balls, Subwassertang

Add the topsoil, clay and sand

First, put about 1-1.5" of the topsoil in the tank. You can create a back to front sloping effect with the back having 1.5" topsoil and the front 1" topsoil. Press it gently so it lets out the air. Next, bury the clay balls in the topsoil, spread apart 2" from each other.

Now, add the sand up to an inch.

Add a little water

Add water up to 3" very slowly. For best results, syphon dechlorinated water from a higher level into the tank using an airline tube. Place the lower end of the airline tube on a rock or a hard surface in the aquarium so that the incoming water will not disturb the sand or soil.

Add the plants

Add the plants according to your aquascape design. Keep the vals in the back, and the stems in the back and middle. The crypts can go in the middle or front. Tie the Java ferns and Anubias to rocks or driftwood; don't let their rhizomes get buried in the substrate.

Fill the tank with water

Now, fill the rest of the tank with water. Fill it slowly, using the syphon method as described above. If you have a larger tank (larger than 20 gallons), wait till the tank has been filled up about 8" and then gently pour water in using a clean cup. Make sure the water does not disturb the sand or soil.

If the water disturbs the sand or soil and you see it kicking up, do not worry. It usually would take a while to settle and clear up, sometimes overnight or 24 hours.

Add snails

Add a few Malaysian trumpet snails. These little guys will live and walk in the substrate, and occasionally come out to say hi. They aerate the soil, and that's very good for the ecosystem you're building.

Fishless Cycling begins

You will have to wait for about 3-4 weeks for the nitrogen cycle to kick in. You can jump start the nitrogen cycle by adding a raw shrimp or fish for 12 hours. That introduces ammonia to the tank. If you don't like the idea of adding dead shrimp or fish, add a little fish food to the water.

Remember, the nitrogen cycle is: Ammonia -> Nitrite -> Nitrate

Do 25% water changes every 3 days. Test for ammonia, nitrite and nitrates.

The water will initially become cloudy - that's the sign of beneficial bacteria present in your tank.

At some point, when ammonia and nitrites are 0, and nitrate is under 40, you're all set to introduce fish in the tank.

Add Fish

Introduce danios and/or guppies, few at a time. Danios are very hardy guys. Guppies are lovely to look at, and in our tanks, breed like crazy.

Algae Control

We added a Tiger Nerite snail to take care of algae, and it cleaned up part of the tank overnight. Certain parts of the tank looked very clean the next day. The ramshorns that hitchhiked on plants we got from a planted tank forum have also been doing their job in algae control, apart from eating dead and rotting leaves.

Water Movement

Water movement is always a good thing, even if it's not required. We added two air stones on opposite sites for backup oxygen and minimal water movement. If you have dead spots, you run the risk of increasing algae in that area. At one point, we tried using a Koralia Nano 240 powerhead, but it was too powerful for this little tank. In future, we plan to use a DIY waterfall with PVC pipes and air tubing.

Problems encountered

As of March 14, 2015, the only problem was a little black beard algae (BBA). It appeared on the anubias coffeefolia leaves. To get rid of it, I got rid of the affected leaves and rubbed algae off the rest of the plant. Increasing water movement could also help.

Snails are not a problem for us, in fact we love snails. They are very useful for aeration and algae control, and fun to watch! If you have excess snails, it is a sign of overfeeding and the solution is to cut down on feeding frequency and amount. Our snails did not breed very fast, and we sold or gave away the excess snails.


Once your tank is set up, the only routine thing for you to do is top-off water every other week. Also, be on the lookout for behavioural changes in the fish. If the fish get sick or appear sluggish, do a 10-20% water change. Avoid large water changes or else you'll destroy the balance in minutes and end up with dead fish/snails.

Our 20 gallon Walstad tank has been running for a year now with water top-offs. The first 6 months, we didn't do any water changes. After that, we changed 10% of the water three times only because the soil got kicked up while rearranging the plants. In 2015, we only made one water change of 10%. Everything is thriving in it.

It would be a good idea to test the water every few months, just in case.


July 2016:

  • We moved to a different house on July 1, 2016. We tore down this 20 gallon tank and another 10 gallon shrimp tank. We gave away most of the plants and accessories. We kept a few of the plants, lava rocks and red cherry shrimp to use in our new house.

We set up a 2 gallon Walstad jar in our new house on July 12, 2016.

June 2015:

  • The plants continue to grow without fertilizer or anything. We sold and gave way Amazon Frogbit, Subwassertang and Rotala Indica/Rotundafolia plants.

May 2015:

  • This tank has become a guppy tank. We donated all six danios to a local hobbyist. The next day, we bought 6 new guppies - red tuxedo and red mutt guppies (sold as fancy "guppy").
  • The only female guppy that we had died on Mother's Day. She had several batches of babies in the last year, some of which turned out to be snakeskin guppies and metallic snakeskin guppies. Before we bought her from PetSmart, she was obviously busy!


We are not scientists or biologists. We just started this 20 gallon Walstad tank as an experiment to see how it would work as a self-sustaining ecosystem and it worked great. We set up another aquarium - 10 gallon - with similar results. Your mileage may vary.

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Last Updated: June 30, 2016.     This post was originally written on March 07, 2015.