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Aquarium cycling basics before buying fish

A beginner-friendly guide to filter bacteria, testing and stocking patience.

7 min readLast reviewed 1 July 2026Education reviewed

This advice is educational and does not replace a qualified veterinarian. Seek professional advice for illness, injury, specialist diets or sudden behaviour changes.

Cycle first, stock later

A filter needs time to establish beneficial bacteria that process waste. Test water before adding fish and stock slowly.

Warning: Clear water is not proof that a tank is safe.

Maintain the filter carefully

Rinse media in old tank water, not untreated tap water, and avoid replacing all media at once.

Sources

  • Practical Fishkeeping cycling guidance
  • OATA fish welfare information

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