How Long To Cycle A Fish Tank
without the guesswork.
Fishless Aquarium Nitrogen Cycle Timeline — Decide fishless-cycle readiness from elapsed time, the ammonia/nitrite spike pattern and current water tests. Includes a practical guide and embedded calculator. Inputs stay in your browser.
RSPCA guidance makes the test pattern—not a fixed countdown—the decision gate: ammonia and nitrite rise, then both return to zero after the tank has run for at least two weeks.
Context matters.
- RSPCA recommends testing water every other day during cycling and notes that there is no exact universal duration.
- A reputable aquatic retailer can independently test a water sample before fish are added; introduce livestock gradually and match temperature and water chemistry to the species.
- Do not add fish while ammonia or nitrite is detectable.
Sources & scope
This page shows the source trail, jurisdiction and review date alongside the result. Always confirm time-sensitive rules with the linked authority.
What to know before using the result.
How do I know when a fishless cycle is complete?
RSPCA guidance says ammonia and nitrite should first spike and then both return to zero. The tank should also have run for at least two weeks; elapsed days alone do not prove readiness.
Why does CalcSpan ask whether fish are already present?
Adding an ammonia source to a tank that already contains fish can harm them. A fish-in situation needs prompt water testing and experienced aquatic-care guidance, not this fishless model.
Does nitrate have to be above zero?
Not in this calculator. Nitrate is shown as context because plants and water changes can alter it; the cited readiness pattern is based on ammonia and nitrite.