The short answer
Cycle a new aquarium before adding fish. The aim is to establish a biological filter that can process the waste your planned stock will produce. Running an empty tank for a fixed number of days, or waiting until the water looks clear, does not prove it is ready.
Use a documented fishless-cycling method and record the results. Your method should include a measured ammonia source, ammonia and nitrite testing, and a final check of processing capacity. Do not dose ammonia into an aquarium containing fish or other livestock.
What the nitrogen cycle does
Fish waste and decaying material produce ammonia. Microorganisms supported by the filter and other aquarium surfaces convert ammonia to nitrite, then nitrate. Ammonia and nitrite are harmful to fish; nitrate still needs managing. Read our aquarium filtration guide for the difference between biological and mechanical filtration.
Prepare the tank before starting
- Install the filter, substrate and equipment, checking the manufacturer's operating instructions.
- Treat source water appropriately before filling and record its pH, hardness and temperature.
- Keep the filter running with suitable circulation and aeration.
- Use an aquarium fishless-cycling ammonia product with known concentration and current instructions, not a household cleaning product.
- Have ammonia and nitrite tests available; check exactly what units they report.
Follow one method, not several dosing recipes
Different ammonia products have different concentrations. A dose copied from another bottle may be wrong for yours. Use the estimated actual water volume, accounting for substrate and equipment, rather than blindly using the tank's advertised capacity.
DrTim's fishless-cycling instructions illustrate why both ammonia and nitrite matter: repeatedly adding ammonia while nitrite is already high can delay the process. Follow the chosen method's testing, redosing and water-change directions. Do not keep adding ammonia solely because its reading has fallen.
Use milestones instead of a 30-day promise
- Set up and record a baseline. Note the ammonia product, amount used, water volume and test-kit units.
- Track processing. Record ammonia and nitrite as directed by the method. A changing result is more useful than the calendar.
- Investigate a stall. Check dose history, test technique, pH, temperature and filter operation before adding more products.
- Confirm readiness. Complete the method's final measured-dose check, then prepare the water for the intended species before adding fish.
Mature media from a healthy aquarium or a suitable bacterial starter may help establish filtration, but neither replaces testing. Do not transfer media from a tank with a suspected disease problem.
How do I know the tank is ready?
In a properly followed fishless-cycling method, the final test checks whether the prescribed ammonia dose is processed without ammonia or nitrite remaining at the specified check time. This is not the same as taking one zero reading from water that has never received an ammonia source.
Nitrate alone is not proof of readiness: source water may contain it, and plants may use it. Check nitrate against the requirements of your planned fish and adjust with appropriately prepared water changes. Our water-test guide explains how to read the results together.
If fish are already in an uncycled tank
Do not add ammonia. Pause further stocking, test the water and keep filtration and aeration working. Detectable ammonia or nitrite requires prompt action: use properly conditioned, temperature-matched partial water changes suited to the readings and the animals, then retest. Ask an aquatic professional for help if results persist or fish are distressed. Do not add salt, medicines or multiple additives by guesswork.
Common questions
Will the aquarium always be ready in 30 days?
No. The starting media, method, water conditions and test results determine progress. Do not book a fish delivery around a guaranteed cycling date.
Does cloudy water mean the cycle is finished?
No. Cloudiness cannot confirm the filter's capacity or water safety. Use our cloudy-water checklist rather than treating appearance as a test.
What happens after the first fish arrive?
Introduce a suitable stocking group only when the system can support it, then monitor the fish and water closely. Follow a species-specific acclimation process and maintain the filter without replacing all established media at once.