This guide goes into more depth than our introductory Getting Started: Setting Up an Aquarium guide – use our Aquarium Volume Calculator and Aquarium Stocking Calculator alongside this guide.
The nitrogen cycle
A new aquarium needs to be “cycled” before fish are added – this means establishing a colony of beneficial bacteria that convert toxic ammonia (from fish waste and uneaten food) into nitrite, and then into far less harmful nitrate. Adding fish to an uncycled tank is a very common beginner mistake and can be fatal to fish due to ammonia and nitrite poisoning.
- Cycling commonly takes several weeks, using either a “fishless cycling” method (adding an ammonia source without fish present) or a slow, carefully monitored “fish-in” cycle with very light stocking and frequent water testing.
- Test ammonia, nitrite and nitrate regularly during cycling – a tank is generally considered cycled once ammonia and nitrite both consistently read zero after a period of stability.
Filtration
An appropriately sized filter for your tank volume is essential – it provides the surface area where the beneficial bacteria from the nitrogen cycle live, as well as removing physical debris from the water. Common types include internal filters, external canister filters, and sponge filters, each suited to different tank sizes and fish types.
Water testing
A liquid-reagent test kit is generally considered more accurate than test strips for tracking ammonia, nitrite, nitrate and pH. Regular testing – more frequently while a tank is new or recently restocked, less often once established – helps catch problems before they affect your fish.
Water changes
Regular partial water changes (commonly cited as around 10-25% weekly for an established, appropriately stocked tank, though this varies by bioload and tank type) help keep nitrate and other dissolved substances at safe levels, since even a cycled tank’s bacteria do not remove nitrate the way they remove ammonia and nitrite. Always treat tap water with an appropriate dechlorinator before adding it to the tank.
Choosing fish and stocking
Research the adult size, temperament, and specific water parameter needs of every species before buying – what looks like a small fish in a shop may grow considerably, and not all species are compatible with each other. Our Aquarium Stocking Calculator gives a very rough starting estimate only – always research actual species compatibility and needs on top of that.
Frequently asked questions
How long does it take to cycle a new aquarium?
Commonly cited as several weeks, depending on the method used and how consistently ammonia and nitrite are being tested and managed.
Can I add fish straight away in a new tank?
This is a very common beginner mistake and is generally not recommended – an uncycled tank can expose fish to dangerous ammonia and nitrite levels.
How often should I do water changes?
Commonly cited as around 10-25% weekly for an established, appropriately stocked tank, though this varies by bioload, filtration and tank type.
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