The nitrogen cycle in aquariums: cycling without losses

The nitrogen cycle is the main biological foundation of an aquarium. Understanding it eliminates 80% of fish-loss problems beginners run into.

What happens in an aquarium

Fish excrete ammonia (NH₃) — a highly toxic substance. In nature it is diluted by huge volumes of water; in an aquarium, its concentration quickly becomes lethal.

Bacteria of the Nitrosomonas genus convert ammonia into nitrite (NO₂⁻) — almost equally poisonous. Nitrobacter (and Nitrospira) then convert nitrite into nitrate (NO₃⁻) — relatively safe for fish at moderate concentrations.

Full cycle: NH₃ → NO₂⁻ → NO₃⁻. Nitrate accumulates and is removed by water changes and absorbed by plants.

Cycling time

Nitrosomonas bacteria multiply in 7–14 days. Nitrobacter — in 14–21 days. The full cycle establishes in 3–4 weeks given a source of ammonia.

Fishless cycling

The most humane method. Every 2–3 days add ammonia (household solution without surfactants) up to 2–3 mg/L. Test with strips or liquid kits. The cycle is complete when ammonia and nitrite read 0 and nitrate is accumulating.

Cycling with test fish (deprecated)

Adding 1–2 hardy fish (zebra danio). Modern aquarium ethics rejects this — fish endure severe stress. Fishless cycling is better.

Signs the cycle is done

Tests show:

• Ammonia (NH₃/NH₄⁺): 0 mg/L

• Nitrite (NO₂⁻): 0 mg/L

• Nitrate (NO₃⁻): 10–40 mg/L

After a week of stable zeros on the first two — you can add fish. In phases, never all at once.

What speeds up the cycle

• Bacterial starters (Tetra SafeStart, Seachem Stability) — contain live cultures. They shorten the cycle to 1–2 weeks.

• Using some substrate or filter media from a running aquarium.

• Temperature 26–28 °C — bacteria are more active.

• Good aeration — nitrifying bacteria are aerobic.

What slows or breaks the cycle

• Any antibiotics and most algicides.

• Chlorine/chloramine in tap water — kills bacteria. Use a conditioner.

• Hot water input (>32 °C).

• Rinsing the filter under the tap — chlorine kills bacteria. Rinse only in aquarium water.

The safest strategy is to cycle the tank without fish, with an ammonia source, and wait four weeks. The money saved on "test fish" pays for itself many times over later.