Why New Aquarium Soil Releases Ammonia

Written 2026-10-23

Fresh planted-tank soil raises ammonia and nitrite even if you don’t add a single fish — the fertilizer and organic matter packed into new soil leach into the water on their own. Running a standard 60cm tank in the Water Kiwoogi simulator with only fresh soil and no fish for 30 days, nitrite still climbs to 1.85ppm on a hobby test kit — above the roughly 0.8ppm toxic threshold. That’s why a soil tank needs a cycling period before any fish go in at all.

The soil itself is an ammonia source

Fresh planted soil is loaded with fertilizer for root-feeding plants. From the moment it’s poured in, nitrogen compounds in it start leaching into the water — in Water Kiwoogi’s catalog, 34mg of nitrogen across the whole tank leaches out over 14 days. That’s a source completely separate from fish waste — it happens even with zero fish in the tank. Inert substrates like black sand or gravel sit at zero on this number, which makes soil a trap of its own.

Smaller tanks get hit harder

The total amount of ammonia fresh soil leaches is a fixed number, no matter the tank size. But less water means the same amount dissolves into a smaller volume, so the concentration ends up higher. Same fresh soil, no fish, 30 days:

The 30cm cube holds less water than the standard 60cm tank, so its nitrite peaks at more than double. The smaller the tank, soil alone gets more dangerous.

Adding fish at the same time is more dangerous

Pour fresh soil into a standard 60cm tank and drop in 6 neon tetras on day one, and the soil’s ammonia stacks on top of fish waste: nitrite spikes to 9ppm. Fish start dying on day 16, and all 6 are gone by day 17 — far worse than the 1.85ppm nitrite peak from soil alone.

Waiting alone isn’t enough

You might think: wait about two weeks until the soil’s ammonia leach is mostly spent, then add the fish. Running the same tank with the same 6 neon tetras added on day 16 instead (after most of the soil’s leach period is over) changes nothing — nitrite still climbs to 5.02ppm, deaths start on day 21, and all 6 are dead by day 22. Just counting days doesn’t work, because the small amount of ammonia the soil leaches on its own isn’t enough to grow a bacteria colony big enough to handle 6 fish. Waiting by itself isn’t the same thing as cycling.

Why reused soil is safer

Swap in reused (already-leached) soil instead of fresh soil under the same conditions, and the picture changes completely. Reused soil brings an already-established bacteria colony with it, and what little ammonia is left to leach is about a tenth of fresh soil’s (4mg over 3 days). Pour reused soil into a standard 60cm tank and add 6 neon tetras on day one, and nitrite peaks at only 0.49ppm — cycling finishes by day 9, and all 6 fish survive.

So what should you do

If you’re setting up a soil tank, mixing in even a thin layer of soil from an established tank is the safest option — it brings the colony with it. If you have to use fresh soil only, either finish cycling with no fish first, or add a bacteria starter on day one and test ammonia and nitrite with a kit, raising the stock slowly only once readings hold at zero. Simply counting off days isn’t safe. If fish gasp at the surface or look sluggish, do a partial water change right away to bring the concentration down, and talk to a fish store or a vet if you’re worried.

The button below is the standard 60cm tank · reused soil · 6 neon tetras added on day one tank from this guide. Swap the substrate for fresh soil and run it again to see how much more dangerous the ammonia gets.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.