Walstad Tank With Corydoras: Soil & Nitrogen Cycle

A Walstad setup with corydoras is a lovely combination — but you're right to pause and ask about the nitrogen cycle, because soil substrate does change things in ways that trip people up. The organic matter in potting soil releases ammonia as it breaks down, which actually kick-starts a cycle, but it also produces gases and tannins that can cause dramatic water chemistry swings in the first few weeks. The good news is that a heavily planted Walstad tank, once established, largely self-regulates nitrogen through plant uptake — but 'once established' is doing a lot of work in that sentence. You still need to monitor ammonia and nitrite closely during the first four to six weeks, because corydoras are bottom-dwellers sitting right on that substrate, and they'll be the first ones affected if ammonia spikes. Are you starting the tank from scratch, and have you already chosen your soil and cap?

That's a sensible worry and it's exactly the right thing to be thinking about. The key is that initial ammonia surge from the soil is actually a one-time event — it peaks in the first week or two, then the bacterial colony and your plants catch up and it drops. What you want to avoid is adding fish during that peak. With a 20 litre tank, the water volume is small, so swings will be sharper and faster than in a larger tank. Capping with sand is the classic approach and it does slow gas release, which helps. The thing I'd emphasise most: don't rely on your eyes to judge when it's safe. You need to be testing ammonia, nitrite, and nitrate every couple of days through that cycling window. Once you see ammonia and nitrite both hit zero and nitrate is rising, you're cycled. Only then do corydoras go in — and even then, add them gradually.

The API Freshwater Master Test Kit is exactly what I'd tell you to buy, and for a Walstad cycle it's genuinely the right tool. It tests ammonia, nitrite, nitrate, and pH — all four of the parameters you need to track during cycling and beyond. The liquid drop tests in this kit are significantly more accurate than dip strips, which matter when you're trying to detect whether ammonia is at 0.25 ppm or 0.5 ppm — a difference that's invisible on a strip but meaningful for corydoras. It's also cost-effective per test, which matters because you'll be testing frequently for weeks. One specific tip for Walstad tanks: also monitor pH, because decomposing soil can push it down, and corydoras prefer a neutral to slightly acidic range of around 6.5–7.5 — so the pH test in that kit isn't just a bonus, it's genuinely useful here.

Plant it heavily from day one — that's actually central to how a Walstad tank works. The plants compete directly with bacteria to process ammonia, and fast-growing stem plants like hornwort or water wisteria can absorb ammonia almost as fast as it's produced. Starting with sparse planting defeats the whole purpose of the method and leaves you with a soil tank that's gassing ammonia with nothing to absorb it. Aim for roughly 70–80% of the surface covered with plants when you fill it. Floating plants are especially valuable in the early weeks because they grow quickly and pull nutrients hard. Then test with your API kit every two to three days, do small water changes if ammonia climbs above 1 ppm, and wait for that clean zero-zero reading before the corydoras go anywhere near it.

Corydoras are shoaling fish and genuinely stressed when kept in small numbers — I'd say four is the realistic minimum for their wellbeing, but in a 20 litre tank that's also close to your sensible stocking limit if you want the Walstad system to stay balanced without regular intervention. Start with four, let the tank settle for another couple of weeks, test again, and only consider adding more if your readings stay clean. Pygmy corydoras are worth considering specifically for a 20 litre — they stay smaller, produce less waste, and work well in planted tanks. Sterbai or bronze corydoras are lovely but better suited to 40 litres or more. Get the species decision right before you buy, and you'll have a much easier time keeping the nitrogen stable.