Aquarium · Tank cycling
Nitrogen cycle timeline calculator
Cycling is the process of growing enough of the right bacteria in your filter to convert fish waste from toxic ammonia into nitrite, then into much less harmful nitrate. Adding fish to an uncycled tank is the single most common way new aquarium owners lose fish in the first month, because ammonia and nitrite are both lethal at the levels an uncycled filter lets build up. This calculator gives you a realistic day-by-day window for your specific setup so you know roughly when it's actually safe to add fish, not just a generic "a few weeks" answer.
Estimated cycling window
27–34 days
What to actually buy
API Freshwater Master Test Kit
This whole timeline is an estimate. The only way to know your actual ammonia, nitrite, and nitrate levels day to day is to test the water directly, and this is the test kit the cycling guidance above is written around.
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How this is calculated
The baseline timeline comes from published fishless-cycling guidance: roughly 27 to 34 days for ammonia and nitrite to spike and fall back to zero at 78°F and a pH between 7.0 and 8.5.
That baseline is then adjusted for your inputs. Nitrifying bacteria (Nitrosomonas and Nitrobacter) roughly double their growth rate for every 10°C of warming, so cooler water stretches the timeline and warmer water (up to about 86°F) shortens it. pH below 7.0 measurably slows the same bacteria, so lower pH also stretches the estimate. Seeding with mature filter media speeds things up considerably, and fish-in cycling runs slower and carries more risk to the fish.
This is an estimate, not a guarantee. Test your water directly with a liquid test kit rather than relying on the calendar alone.
Sources
Frequently asked questions
Can I add fish before the cycle finishes?
Not safely. Ammonia and nitrite are both toxic to fish at levels an uncycled filter allows, which is why fish-in cycling (this calculator's slowest option) exists as a fallback for people who already have fish in an uncycled tank, not a recommended starting method. If you're setting up a new tank, wait for two consecutive zero-ammonia, zero-nitrite readings before stocking.
Why did my ammonia spike again after it dropped?
That's usually the nitrite spike doing exactly what it's supposed to do. Nitrosomonas bacteria convert ammonia to nitrite first, and a second bacteria colony (Nitrobacter) has to build up separately to convert that nitrite to nitrate. A temporary ammonia rebound partway through is common and doesn't mean the cycle restarted, keep testing rather than re-dosing.
Does an air stone or extra filtration speed up cycling?
Indirectly. Nitrifying bacteria need oxygen to do their work, so good surface agitation and flow through the filter media help them establish faster. It won't override the temperature and pH effects this calculator accounts for, but a stagnant, poorly oxygenated tank will cycle slower than the estimate above.
What if my tap water already has some ammonia or chloramine?
Use a dechlorinator that neutralizes chloramine (most do) before dosing ammonia for a fishless cycle, otherwise you're fighting chlorine's antibacterial effect against the exact bacteria you're trying to grow. If your tap water tests positive for ammonia after dechlorinating, that's a different issue worth addressing with your water utility, not something this calculator adjusts for.