Aquarium cycling step by step

The nitrogen cycle converts toxic waste (ammonia → nitrite → nitrate) thanks to nitrifying bacteria in the filter and aquarium surfaces. Mounting fish before the cycle is established is a common cause of mortality.

What is the cycle (useful summary)

Reference:Merck — aquarium fish

1. Ammonia (NH₃/NH₄⁺):very toxic; It comes from feces, urine, leftover food and organic matter.
2. Nitrite (NO₂⁻):also toxic; It is produced by bacteria that oxidize ammonia.
3. Nitrate (NO₃⁻):less acute, controlled with water and plant changes/maintenance.

How to tell if it is cycled

• Use reliable aquarium tests (liquid kit preferable to dubious strips).
• Typical practical goal in established freshwater: ammonia and nitrite undetectable or at kit minimum; controlled nitrates with partial changes.
• Cycling time varies (often weeks). There is no universal magic calendar.

What to do when riding

• Install the appropriate filter, temper, and condition the water according to the product and type of aquarium.
• Fishless cycling with controlled ammonia source, or very gradual loading only with experienced judgment.
• Do not wash the filter material with hot tap water: you destroy the bacterial colony.

Alarm signs in fish

• Gasping at the surface, lethargy, stuck fins, sudden death after a change or overfeeding.
• Ammonia/nitrite elevated in the test.

What not to do

• Don't add a large load of fish on day 1.
• Do not "medicate" blindly or use network remedies without a diagnosis.
• Don't mix dog/cat advice in water maintenance.

Frequently asked questions