How to Cycle a New Aquarium Before Adding Fish

The goal of cycling is to grow two bacterial colonies

The goal of cycling is to grow two colonies of bacteria: one that converts toxic ammonia into nitrite, and another that converts nitrite into far less harmful nitrate. Until both colonies are established, ammonia or nitrite can quickly reach levels that harm or kill fish. Cycling simply means feeding these bacteria an ammonia source and giving them time to multiply until they can keep pace with a stocked tank.

The fishless cycle is the kindest, most popular method

The kindest and most popular method is the fishless cycle. Instead of subjecting fish to toxic water, you add an ammonia source yourself, either bottled pure ammonia or a pinch of fish food left to decay. This creates the ammonia the bacteria need to colonize your filter and substrate. Keep the tank running with the heater and filter on throughout, because the bacteria need warmth, oxygen, and flow to establish.

Testing the water tracks the cycle's progress

Testing the water is how you follow the cycle's progress, so a reliable test kit that measures ammonia, nitrite, and nitrate is essential. At first you will see ammonia climb. After a while nitrite will appear and rise as the first bacteria get to work, and ammonia will begin to fall. Eventually nitrate appears as the second colony develops, and both ammonia and nitrite drop back toward zero.

Seeding can speed up the process

You can speed things up by seeding the new tank with mature media, such as a used filter sponge or some substrate from an established, healthy aquarium. This introduces living bacteria directly rather than waiting for them to arrive on their own, and it can shorten the cycle considerably. Bottled bacteria products can help too, though results vary. Patience remains important either way; a full cycle commonly takes several weeks.

Knowing when your tank is fully cycled

You know the tank is cycled when it can process ammonia rapidly: add an ammonia source and, within about a day, both ammonia and nitrite read zero while nitrate is present. At that point do a large water change to bring the nitrate down, and you can begin adding fish slowly. Stock gradually rather than all at once, so the bacterial colonies can grow to match the rising bioload.

Frequently asked questions

What is the fishless method of cycling an aquarium? Adding an ammonia source yourself, either bottled ammonia or decaying fish food, to feed the bacteria that need to colonize your filter and substrate, without subjecting live fish to toxic water.

How can I speed up cycling a new aquarium? Seed the new tank with mature media, such as a used filter sponge or substrate from an established, healthy aquarium, which introduces living bacteria directly and can shorten the cycle considerably.

How do I know when my new aquarium is fully cycled? Add an ammonia source and check that within about a day both ammonia and nitrite read zero while nitrate is present, then do a large water change before adding fish gradually.

Should I add all my fish at once after cycling a new tank? No, stock gradually rather than all at once, so the bacterial colonies can grow to match the rising bioload as you add more fish.