The Walstad method — soil under a sand cap, low light, no CO2, plants as the primary filter — works beautifully in nano tanks when you scale it correctly. I have been running a 10 gallon Walstad on my desk since 2022, a 5 gallon shrimp Walstad since 2023, and a 15 gallon Walstad community since 2024, and the small versions are the most rewarding tanks in my fishroom. They are also the tanks I get the most questions about, because the standard Walstad advice (deep soil, big schools of fish, weekly water changes) does not scale down linearly. This guide is what I have actually done in tanks under 15 gallons. For the broader Walstad philosophy, read the full Walstad Method Guide first — this page assumes you understand the basics and focuses on what changes when the tank is small.
Under-stock, over-plant, and leave it alone. A 10 gallon Walstad is not a 75 gallon Walstad scaled down by a factor of 7.5. It is a different beast with different stability characteristics, and the path to success is to put less in it than you think you can. Ten chili rasboras, six pygmy corys, and a colony of cherry shrimp in a heavily planted 10 gallon Walstad is a tank that runs itself for years. Twenty tetras and a bristlenose in the same tank is a tank that crashes in three months. The math is brutal because the water volume is small.
Why Walstad for Nano?
The Walstad method shines in nano tanks for two reasons: it eliminates the equipment clutter that dominates small-tank setups, and it produces a level of parameter stability that no filter-only nano tank can match. A traditional 10 gallon nano tank needs a hang-on-back filter, a heater, an air pump, a CO2 system if you want plants to grow, and a strict weekly water change schedule just to keep the parameters from swinging. A 10 gallon Walstad needs a small sponge filter (or just an air stone), a heater if you keep tropical fish, and water changes measured in months, not weeks. The complexity drops by an order of magnitude.
The parameter stability is the bigger win. Soil under a sand cap acts as a buffer for ammonia, nitrate, pH, and trace minerals — the soil's cation exchange capacity holds nutrients and releases them slowly, the plants consume nitrogen compounds as fast as the fish produce them, and the resulting tank holds parameters steady for weeks at a time. In a 10 gallon filter-only tank, a single overfeeding can spike ammonia to 0.5 ppm overnight. In a 10 gallon Walstad, the same overfeeding is absorbed by the soil and plants and reads as zero on the test the next morning. That stability is what lets you keep delicate nano fish — chili rasboras, celestial pearl danios — in a tank you would not dare put them in otherwise.
The trade-off is the first 30 days. A Walstad nano tank is a construction site for the first month — soil releasing ammonia, algae taking advantage of the nutrients, plants not yet rooted, water chemistry swinging. This is the wall that new Walstad keepers hit, and it is the wall that makes Walstad "intermediate" rather than "beginner" difficulty. If you can survive the first 30 days, the next 30 months are easy. If you panic during the algae phase, tear the tank down, and start over, you will repeat the cycle until you quit the hobby.
The Soil Layer: Miracle-Gro Organic, 2.5 cm
The soil is the engine of a Walstad tank, and the soil you choose matters more than anything else. The hobby standard is Miracle-Gro Organic Choice Potting Mix — it has been tested by tens of thousands of Walstad keepers over more than a decade, it has a known nutrient profile, and it does not contain the synthetic slow-release fertiliser beads that cause algae explosions in newer potting mixes. Sift it through a half-inch mesh to remove the largest wood chips and any perlite (the white balls — they float, and you will be picking them out of your water column for weeks if you leave them in). The sifted soil goes in damp, not wet, in a layer 2.5 cm thick.
Why 2.5 cm? In a nano tank you may be tempted to thin the soil layer to leave more water volume, but a thinner layer (under 2 cm) exhausts its nutrients within a year and the plants start to yellow. A thicker layer (over 4 cm) goes anaerobic at the bottom and produces hydrogen sulfide — the rotten-egg smell that means bacteria are respiring without oxygen. Two and a half centimetres is the sweet spot: enough organic matter to feed root-feeding plants for two to three years, shallow enough to stay oxygenated at the bottom through diffusion. In a 5 gallon tank, 2.5 cm of soil is about 1.5 litres of sifted mix — one bag of Miracle-Gro will set up three nano tanks.
Alternatives if you cannot find Miracle-Gro Organic: Sta-Green Organic Potting Mix (similar formula, similar results), any unscented unfertilised organic topsoil from a garden centre, or purchased Walstad-specific soil blends from specialty suppliers. Avoid anything with perlite (floats), vermiculite (releases too much ammonia), moisture-control crystals (they swell and push the soil up), or synthetic fertiliser (the blue or green beads that scream "this will dump nitrogen and cause algae"). If the bag says "feeds for 3 months" or "feeds for 6 months," do not put it in your tank — you want soil, not fertiliser.
The Cap: Sand or Fine Gravel, 2.5 cm
The cap is what keeps the soil out of your water column. Without it, every fish that digs, every plant you uproot, every snail that burrows will release a puff of soil into the water and cloud the tank for hours. The cap also provides the rooting medium for plants that prefer sand or gravel over soil (Anubias, Java fern, Bucephalandra — though these are epiphytes and should be tied to hardscape, not planted in the cap). Use 2.5 cm of fine sand (pool filter sand, play sand, or CaribSea Super Naturals) or fine gravel (2–3 mm grain size) as the cap. Do not use coarse gravel — the gaps let soil leak through.
The cap depth matters in a nano tank. A cap under 2 cm lets soil leak when fish dig — pygmy corys are relentless substrate sifters, and a thin cap will give them access to the soil layer underneath, clouding the water and exposing anaerobic pockets. A cap over 4 cm is wasted depth that does nothing but reduce your water volume. Two and a half centimetres is the minimum that stands up to digging fish and the maximum that makes sense in a small tank. If you want to keep digging species (corydoras, loaches), go 3 cm and skip the soil at the front of the tank where they will be most active.
Wash the cap material before adding it. Sand and gravel from the hardware store carry dust that will cloud your tank for days if you do not rinse them. Half-fill a bucket with the cap material, run a hose into it, stir, pour off the cloudy water, repeat until the water runs clear. This takes 15 minutes and saves you a week of cloudy tank. Once the cap is washed, add it slowly — pour it over a plate or a plastic bag on top of the soil to avoid disturbing the soil layer. The cap goes on dry or damp, never wet, and you fill the tank by pouring water onto a plate at the surface to avoid disturbing the layers.
Plant Selection: Heavy Root Feeders
The plants you choose for a Walstad nano are different from what you would choose for a high-tech tank. You want heavy root feeders — plants that pull nutrients from the soil through their root systems — because the soil is where the nutrients are. Stem plants (Rotala, Ludwigia, Hygrophila) are water-column feeders that will not use the soil effectively in a low-tech tank, and they need more light than a Walstad nano provides. Stick to the root feeders: Vallisneria, Cryptocoryne, Echinodorus (sword plants), and the larger-rooted carpets like Staurogyne repens.
Vallisneria is the workhorse of the Walstad nano. It grows fast, it sends runners across the substrate, and it pulls enormous amounts of nitrogen from the water column through its root system. A single pot of Vallisneria spiralis planted in a 10 gallon Walstad will be a forest of 50 plants in six months, and that forest is what keeps the ammonia reading at zero. Plant it in the back third of the tank — it grows tall and will shade everything behind it. The other Vallisneria species (corkscrew, asiatica, tortifolia) work the same way and add visual variety.
Cryptocoryne is the second pillar. Crypts come in a huge range of sizes — from the tiny Cryptocoryne parva (3 cm) to the mid-sized wendtii (15 cm) to the large balansae (40 cm) — and they all thrive in Walstad conditions. They are slow growers (expect one new leaf every two weeks per plant), but they are unfussy, they tolerate low light, and they pull nutrients steadily from the soil. The Cryptocoryne "melt" (where the leaves dissolve into mush in the first week after planting) is normal — the plant is adapting to your water parameters and will regrow from the roots in 2–3 weeks. Do not pull a melted crypt; it will be back.
Echinodorus (Amazon sword and the smaller sword varieties) rounds out the trio. A single Amazon sword in a 15 gallon Walstad becomes the centrepiece of the tank — broad leaves, root system that fills the soil, steady growth. In a 10 gallon, use the smaller Echinodorus species (E. bleheri compact, E. quadricostatus) so the plant does not take over the tank. Swords are heavy feeders and will tell you when the soil is running low on nutrients (the leaves yellow and slow their growth around month 18–24) — that is your cue to add root tabs or refresh the soil. For carpet plants, Staurogyne repens works in a Walstad nano if you give it the brightest spot in the tank; everything else (HC Cuba, Glossostigma, dwarf hairgrass) needs CO2 and high light and does not belong in this setup.
Lighting: Low to Moderate, No CO2
Walstad nano tanks run on low to moderate light — the goal is enough light for the plants to photosynthesise, not enough to trigger algae. A 10 watt full-spectrum LED over a 10 gallon, on for 8 hours a day, is plenty. The specific fixture matters less than the consistency: a cheap LED on a timer is better than an expensive fixture on a manual switch, because the plants need a stable photoperiod to thrive. Use a smart plug or a $10 mechanical timer — either works — and set it for 8 hours on, 16 hours off. Do not run the light longer than 10 hours; the marginal plant growth is not worth the algae risk.
Spectrum matters more than intensity in a Walstad. You want a full-spectrum LED (6500K colour temperature, with both red and blue peaks in the spectrum) — these are marketed as "plant grow lights" or "aquarium plant lights" and cost $20–$50 for a 10 gallon fixture. Avoid cheap white-only LEDs (the $10 "fish tank light" on Amazon) — they produce a flat white spectrum that grows algae better than plants. Avoid the deep-blue "reef" lights — they are tuned for coral photosynthesis, not freshwater plants, and they make your nano fish look washed out.
CO2 is not just unnecessary in a Walstad nano — it is actively counterproductive. The whole point of the Walstad method is that the soil and the fish provide all the carbon the plants need through bacterial decomposition and fish respiration. Adding CO2 to a Walstad nano causes two problems: it drives the pH down (sometimes crashing it), and it accelerates plant growth past what the soil nutrients can support, which leads to nutrient deficiencies and the kind of algae that thrives on CO2-rich, nutrient-poor water. If you want to run CO2, run a high-tech tank — do not try to add it to a Walstad and expect the best of both worlds. You get the worst of both.
Fish Stocking for Walstad Nano
The fish stocking for a Walstad nano is dictated by one rule: small bioload only. The soil and plants handle a moderate amount of fish waste, but they cannot absorb the output of a heavy feeder. The fish I trust in Walstad nanos are the small, low-waste species: chili rasboras (Boraras brigittae), ember tetras, celestial pearl danios, pygmy corydoras (C. pygmaeus, C. hastatus, C. habrosus), and a single betta in a 10 gallon or larger. Pair any of these with a colony of cherry shrimp (10–20 in a 10 gallon) or 3–5 Amano shrimp, and you have a complete clean-up crew that works the surfaces the fish ignore.
For a 5 gallon Walstad, stock either a single betta OR a colony of 10–15 cherry shrimp. Not both — the betta will eat the shrimp babies and stress the adults, and the 5 gallon volume does not give the shrimp enough refuge. For a 10 gallon, the classic Walstad nano stocking is 10 chili rasboras + 6 pygmy corys + 10 cherry shrimp. That is roughly 60% of the bioload the tank can support, which leaves headroom for the system to absorb the occasional overfeeding. For a 15 gallon, push it to 15 chili rasboras + 8 pygmy corys + 15 cherry shrimp + 3 Amano shrimp. Use the nano stocking wizard or the stocking calculator to check the bioload numbers before you commit.
Fish that do NOT belong in a Walstad nano: anything that digs (cichlids, larger corydoras species, loaches — they tear up the soil layer and the cap, and you will be cleaning soil out of the water column weekly), anything with a heavy bioload (goldfish, plecos of any size, large barbs — the system cannot absorb the waste), anything that needs high flow (hillstream loaches, rainbow shiners — the Walstad nano is a low-flow tank by design), and anything sensitive to organic compounds in the water (German blue rams and discus do not belong in a soil-based tank, even a mature one). If you want to keep these fish, build a different kind of tank — the Walstad is not the right tool.
The First 30 Days: Algae Phase, Patience
The first 30 days of a Walstad nano are the hardest part of the hobby, and they are the part that makes or breaks new Walstad keepers. The soil is releasing ammonia and dissolved organics into the water column at the same time the plants are not yet rooted and cannot take up those nutrients. The result is algae — diatoms (brown dust on every surface), green water (single-celled algae that turns the water pea-soup green), hair algae (green threads on plant leaves and hardscape), and sometimes blue-green algae (cyanobacteria, technically, that coats the substrate in a slimy sheet). This is normal. This is expected. This is not a reason to tear the tank down.
The protocol for the first 30 days: do 25% water changes twice a week for the first two weeks, then weekly through day 30. Test ammonia every other day — if it reads above 0.25 ppm, do a 50% water change immediately and skip feeding for a day. Leave the lights on for 6 hours a day (not 8) for the first two weeks to limit algae while the plants establish. Manually remove the worst of the hair algae with a toothbrush — twirl it like cotton candy and pull it out. Do not add fish until day 30, and do not add the full stock at once — add half on day 30, the other half on day 45. The tank is not stable until day 60, even if it looks stable.
The algae will peak around day 14–21 and start to recede as the plants establish root systems and begin outcompeting the algae for nutrients. By day 30, the diatoms should be fading, the green water (if it appeared) should be clearing, and the plants should be visibly growing — new leaves on the crypts, runners on the Vallisneria, new growth on the sword. By day 60, the tank should be stable — clear water, growing plants, no new algae outbreaks. If you are still fighting algae at day 60, the issue is either light (too much) or stock (added too early) — reduce the photoperiod to 6 hours and double-check that your fish count is within the bioload the tank can handle.
Maintenance: Rare Water Changes, Top-offs
Once a Walstad nano is established (day 60+), the maintenance schedule is genuinely minimal. Top off evaporated water with dechlorinated tap water weekly — do not skip this, because the parameters concentrate as water evaporates and a top-off restores the balance. Do a 20% water change monthly, siphoning the cap surface to remove detritus without disturbing the soil underneath. Test ammonia, nitrite, and nitrate monthly — if ammonia or nitrite reads above zero, do a 50% water change immediately and investigate (a dead fish, an overturned plant, an aerated pocket of soil). Trim plants as needed — the Vallisneria will need thinning every 2–3 months, the crypts rarely need trimming, the sword will need older leaves removed as they yellow.
Do not over-water-change an established Walstad. The temptation after years of high-tech tank maintenance is to do weekly 50% water changes "to keep things clean." In a Walstad, that strips the dissolved nutrients the plants are feeding on, the plants stop growing, the algae takes advantage of the now-empty nutrient niche, and you end up in an algae spiral that ends with the tank torn down. The whole point of the method is that the system is in balance — the fish produce waste, the bacteria convert it, the plants consume it, and the water stays clean. Breaking that balance with aggressive water changes is worse than doing nothing.
The one maintenance task you cannot skip: filter cleaning. Even a sponge filter accumulates detritus over time, and a clogged sponge reduces water flow and oxygen exchange. Every 4–6 weeks, pull the sponge and squeeze it gently in old tank water (never tap water — the chlorine kills the biofilter) until it runs clear. Replace the sponge when it starts to fall apart — typically every 12–18 months. The air pump diaphragm will also wear out every 18–24 months and need replacing; the symptom is reduced airflow and a louder pump. These are $5 maintenance items that keep the tank running for years.
Common Failures
1. Stocking too heavily in the first 60 days. The tank is not stable until day 60, and adding a full stock of fish on day 30 (or worse, day 1) overwhelms the still-establishing biofilter. The result is an ammonia spike that kills the fish and starts an algae bloom. Add half the stock on day 30, the other half on day 45, and resist the temptation to add "just one more" before day 60.
2. Using the wrong soil. Potting mix with synthetic slow-release fertiliser (the blue or green beads) will dump nitrogen and phosphorus into the water column for months, fueling algae that no amount of water changes will fix. Perlite floats and will give you white foam on the surface for weeks. Vermiculite releases ammonia in pulses that stress the fish. Use sifted Miracle-Gro Organic Choice or a known equivalent — do not improvise on this ingredient.
3. Too much light. Eight hours a day is plenty. Ten hours is the upper limit. Twelve hours (which is what people default to when they want their tank lit "while I am home") is algae fuel. Use a timer, set it for 8 hours, and leave it alone. The plants do not need more light; they need stability.
4. Adding CO2. Walstad tanks do not need CO2, and adding it destabilises the pH and causes nutrient imbalance. If you want a CO2-injected tank, build a high-tech setup — do not try to add it to a Walstad. You get the worst of both worlds: unstable parameters and the maintenance of a high-tech tank without the growth benefits.
5. Tearing down the tank during the algae phase. The 30-day algae bloom is normal. Tearing the tank down and restarting does not skip the algae phase — it resets the clock on it. Ride it out, follow the water change protocol, and the tank will stabilise by day 45–60. Every Walstad keeper I know went through the algae phase; none of them tore the tank down and avoided it.
6. Keeping digging fish in a soil-based tank. Corydoras (the larger species), loaches, and cichlids all dig, and they will dig through a 2.5 cm sand cap into the soil layer underneath. The result is a tank that clouds up weekly and an eventual soil leak that crashes the parameters. Pygmy corys are safe (they sift the cap surface without digging deep), but skip everything else that digs. Use the Walstad soil calculator to verify your layer depths before you build.
Tools and Related Guides
Use the Walstad soil calculator to compute exactly how much soil and cap material you need for your tank volume. For the broader Walstad philosophy and the related guides, start here:
- Full Walstad Method Guide — the general guide, applicable to any tank size
- Walstad Soil Calculator — compute soil and cap volumes for your tank
- Nano Shrimp Hub — cherry shrimp are the ideal Walstad nano clean-up crew
- Nano Stocking Wizard — check bioload for chili rasboras, CPDs, and pygmy corys
- Fish Database — filter by "Nano Fish" to find species appropriate for a Walstad nano
Frequently Asked Questions
Can you do a Walstad tank in a 5 gallon?
Yes. A 5 gallon Walstad works well for shrimp-only or a single betta, but the small volume makes parameter stability harder during the first 30 days as the soil releases ammonia. Use 1.5–2 cm of Miracle-Gro Organic potting soil capped with 2.5 cm of sand, plant heavily with root feeders (Cryptocoryne parva, staurogyne, dwarf sagittaria), and stock with 10–15 cherry shrimp or a single betta. Do not add chili rasboras or other nano fish until day 45 at the earliest.
What soil should I use for a Walstad nano tank?
Miracle-Gro Organic Choice Potting Mix is the hobby standard, sifted to remove the largest wood chips and any perlite. Alternatives include Sta-Green Organic Potting Mix and any unscented, unfertilised organic topsoil. Avoid anything with perlite (floats), vermiculite (releases too much ammonia), synthetic slow-release fertiliser beads (causes algae), or moisture-control crystals. If the bag says "feeds for 3 months," do not use it.
How thick should the soil and sand cap be in a Walstad nano tank?
2.5 cm of soil, 2.5 cm of sand or fine gravel cap. In a nano tank you may be tempted to thin the layers to leave more swimming space, but a thinner cap (under 2 cm) lets soil leak through when fish dig, and a thinner soil layer (under 2 cm) exhausts its nutrients within a year. Keep the 2.5/2.5 ratio even in a 5 gallon — the small tank needs the buffering capacity as much as a larger tank does.
Does a Walstad nano tank need water changes?
Rarely, after the tank is established. The first 30–45 days require 25% weekly water changes to handle the ammonia released by the soil. After the soil stabilises (plants growing, biofilm visible, ammonia reading zero for two consecutive weeks), top off with dechlorinated water and do a 20% change monthly. Over-water-changing a mature Walstad strips the dissolved nutrients the plants are feeding on and can trigger algae.
What fish can I keep in a Walstad nano tank?
Small-bioload fish only: chili rasboras, ember tetras, celestial pearl danios, pygmy corydoras, and a single betta in a 10 gallon or larger. Pair with cherry shrimp or Amano shrimp. Avoid anything that digs (cichlids, larger corydoras, loaches), anything with a heavy bioload (goldfish, plecos), and anything that needs high flow (hillstream loaches). The Walstad nano is a low-energy tank for low-energy fish.