How Long Does Cycling Take — Soil vs. Sand Tanks

Written 2026-09-28

A tank with a fresh aquatic-plant soil substrate cycles on its own — the soil leaches enough ammonia by itself, with no fish and no additives, that our simulator model reaches "test-kit zero" for ammonia and nitrite by day 39. An inert substrate like fine black sand releases no ammonia at all, so cycling never starts unless something else supplies nitrogen — fish or an additive. But once fish are actually in the tank, the substrate turns out to matter far less than one other choice: dose a bacteria starter once and hold off stocking until day 22, and both soil and sand tanks finish cycling 7 days later, on day 29, with all 6 neon tetras alive. Skip the starter and stock on day one instead, and fish start dying on day 15–16 regardless of substrate, wiped out by day 18.

Every comparison below uses the same setup — a 60cm standard tank (about 59L), one large sponge filter, low light for 8 hours, a heater at 25°C, and a 25% water change every week starting day 8 — changing only the substrate and how the fish were added. "Cycled" means ammonia and nitrite both drop below 0.05mg/L as nitrogen (about 0.06ppm NH4 and 0.16ppm NO2 on a standard ion-based test kit) and read as zero.

Soil leaches its own ammonia

According to the game’s own published numbers, fresh soil leaches 34mg of nitrogen (as ammonia) into the whole tank over its first 14 days. Running the simulator on that substrate alone, with no fish and no additives, that leaching is enough to grow a bacterial colony that reaches "cycled" by day 39. During those 39 days nitrite peaked at 1.20ppm on a test kit — above the safe line (about 0.8ppm) — which is exactly why adding fish to a fresh soil tank before that leaching settles down is risky.

Sand releases no ammonia

Fine black sand’s catalog values are all zero — no nutrients, no ammonia leaching. Run the same setup with no fish and no additives and ammonia and nitrite never rise at all over 30 days. Nitrifying bacteria need a food source to grow, so an inert substrate alone never starts the cycling process. Something has to supply nitrogen from outside — fish, or a fishless-cycling ammonia solution.

With fish in the tank, the starter matters more than the substrate

Using the same tank with one large sponge filter, dosing a bacteria starter once on day 1, then adding 6 neon tetras on day 22: both soil and sand tanks finish cycling on day 29 — 7 days after stocking — with all six fish alive. The substrate stops mattering by then because the soil’s own leaching has already finished (it lasts under 14 days) and the starter has already established the colony.

Why stocking on day one is risky

With the same tank and filter but no starter, stocking 6 neon tetras on day one asks a colony that’s starting from almost nothing to handle six fish worth of waste right away. In the soil tank, free ammonia peaked at 0.0167ppm (still under the 0.02ppm safe line), but nitrite spiked to 6.62ppm on a test kit — about 8 times the safe line — and fish started dying on day 16, wiped out by day 18. In the sand tank, ammonia itself crossed the safe line at 0.0274ppm and nitrite reached 5.35ppm; deaths started on day 15, also wiped out by day 18.

Without the starter, delaying stocking gave different results depending on the substrate. In the soil tank, all six fish died whether they went in on day 8, 15 or 22 — the ammonia the soil released kept turning into nitrite, which still peaked at 3.51 ppm (test-kit reading) even when the fish arrived on day 22. In the sand tank, stocking on day 8 lost every fish, but stocking on day 15 kept five of six alive, and day 22 kept all six. With the starter dosed once and stocking on day 22, both substrates keep all six fish alive. In short: with a starter the substrate stops mattering; without one, a soil tank needs a much longer wait.

The button below opens the soil tank with the bacteria starter on day 1 and six neon tetras added on day 22 from this guide.

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