How to Cycle an Aquarium
Setting up a new fish tank is one of the most rewarding experiences in the hobby, but it’s also when the first mistakes are commonly made, like adding fish too soon. Before safely stocking fish in your tank, you must complete an important process known as tank cycling.
What Is the Nitrogen Cycle?
The nitrogen cycle is a naturally occurring process that breaks down harmful ammonia into either nitrites or nitrates. Without cycling your aquarium, concentrations of ammonia would build up to toxic levels in just a matter of days or hours, and your fish could die.
Good, live bacteria that colonize surfaces inside your aquarium, especially in the media of a filter and substrate. This microscopic team of workers comes in two main stages: first, Nitrosomonas bacteria turn deadly ammonia into nitrite (which is still toxic but less immediately dangerous), and then another group of bacteria, known as Nitrobacter, converts the nitrite into nitrate, which is relatively safe in moderate amounts and can be managed through weekly water changes.
The cycling process doesn't happen overnight. These bacterial populations usually take 4-6 weeks to become established enough to handle a full bioload of fish.
Monitoring the cycling process
Water testing is essential for tracking your tank's nitrogen cycle. You'll need test kits for ammonia, nitrite, and nitrate.
What to Expect:
When you first add an ammonia source, ammonia levels will spike. Within a few days to a week, you'll see ammonia dropping as nitrite levels begin to rise.
The nitrite spike typically occurs one to two weeks into the process. This is normal—don't panic. As Nitrobacter bacteria colonies establish themselves, nitrite levels will decrease while nitrate levels increase.
Your cycle is complete when both ammonia and nitrite consistently read zero and nitrates are present at low levels.
Visual Indicators:
While water tests are the most reliable method, you may notice some visual signs of progress:
-
A cloudy bacterial bloom early in the cycle
-
Brown algae (diatoms) forming on surfaces as the tank matures
These are normal signs of a developing ecosystem.
Fishless Cycling
Fishless cycling is the most humane and effective way to establish your nitrogen cycle. Instead of using live fish to produce ammonia, you provide an external ammonia source to feed the growing bacterial colonies.
Ammonia Sources:
Choose one of these options:
-
Pure ammonia from hardware stores (ensure it contains no additives)
-
Fish food
-
Commercial cycling products designed for this purpose
The Process:
Start by adding ammonia to reach 2-4 ppm in your tank. Test daily and redose to maintain around 3 ppm until you detect nitrite appearing.
Once nitrite levels start rising, reduce your ammonia dosing to approximately 2 ppm. This prevents overwhelming the second stage of bacterial development.
Monitor and adjust until both ammonia and nitrite consistently read zero within 24 hours of dosing ammonia. The cycle typically completes in 4-6 weeks, though timing varies based on temperature, pH, and other factors. Warmer water (around 84°F) speeds up the process, while cooler temperatures slow bacterial growth.
Why Choose Fishless Cycling?
This method has two key advantages: no fish are harmed during the process, and you can maintain higher ammonia levels than fish could tolerate, potentially establishing bacterial colonies faster.
Fish-In Cycling
Occasionally, some situations require you to cycle with fish already in the tank – maybe you inherited fish or made a newbie's mistake. It’s not ideal, but you can fish-in cycle with close vigilance and lots of water changes.
Start with very few hardy fish, no more than one small fish per 10 gallons initially. Test your water daily and be prepared to perform water changes whenever ammonia or nitrite exceeds 0.5 ppm. During peak cycling periods, you may need daily water changes of 25-50%.
Use a quality water conditioner that temporarily detoxifies ammonia, giving your biological filter time to develop. Products like Seachem Prime and similar ammonia-neutralizing conditioners provide crucial temporary protection for your fish.
Important: Feed Sparingly
Reduce feeding significantly during fish-in cycling. Less food produces less waste and therefore less ammonia. Your fish can survive on a reduced diet, but they cannot survive ammonia poisoning.
Common Cycling Mistakes
-
Adding fish too quickly – Even after initial cycling, too many fish at once will overload your biological filter and restart the cycle. Stock gradually over weeks or months.
-
Over-cleaning during cycling – Avoid deep cleaning filter media or substrate while cycling. Beneficial bacteria need surfaces to colonize, and excessive cleaning wipes them out before they can establish.
-
pH crashes – Bacteria thrive at pH 7.0-8.0 but slow or stop below 6.5. Maintain stable water chemistry through regular water changes and proper buffering.
-
Using medications during cycling – Antibiotics and strong medications kill beneficial bacteria along with harmful ones. Medicating during cycling may require starting the process over.
Post-Cycling
Just because the cycle is over doesn’t mean you’re done. Just as with any living organism, beneficial bacterial colonies need regular maintenance to stay healthy. Avoid cleaning all your filter media at the same time. You want to ensure that some mature bacteria-colonized media is left to sustain viable bacterial populations.
Use tank water and not chlorinated tap water to clean filter media, as chlorine kills beneficial bacteria on contact. You should also monitor your bioload carefully. Overstocking with too many fish at one time, heavy feeding, or adding certain species can outstrip the capacity of even a mature biological filter. Work your way up to your desired fish stock slowly so the bacteria have time to become acclimated to more waste.
Conclusion
Cycling a fish tank does take time, but with patience, it’s one of the most valuable things you’ll learn in the hobby. Keep in mind that every awesome aquarium began just like this. Take the time to learn and properly cycle, you’ll have healthier fish and fewer issues later.
If you need help cycling your tank or would like more information on cycling aquariums, reach out to the Reefco Aquariums team today!
