Cycling a new aquarium: what a week-by-week study measured
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
"Three to six weeks" is the number that circulates everywhere for cycling a new aquarium, but it mostly rests on accumulated hobbyist experience rather than a published scientific tracking study. A study by McKnight, Szabolcs, Graham and Neufeld, of the University of Waterloo, published in 2025 in ISME Communications, tracked week by week what actually happens inside the biofilter (, doi 10.1093/ismeco/ycaf212). Guidance on filtration equipment is gathered in the fish accessories section.
Contents · 6 sections
A weekly tracking study across three real aquariums
The researchers set up three new home aquariums and collected weekly samples, over twelve weeks, of water, biofilter beads and filter floss, tracking both water chemistry (ammonia, nitrite) and the identity of the microorganisms present.
What the real timeline shows
The concrete result: ammonia and nitrite fell to undetectable levels as early as week 3 in two of the three aquariums, but it took until week 8 in the third — a clear gap between tanks set up under otherwise comparable conditions, which illustrates why a single blanket timeline is misleading.
Who does the work, and in what order
On the microbiology side, ammonia-oxidizing archaea (AOA) preferentially colonised the biofilter beads. Genes from Nitrospira bacteria capable of carrying out complete nitrification on their own (so-called "comammox", or CMX) were detected in all three aquariums by week 12, confirming their role already identified as dominant in mature aquarium biofilters. The authors hypothesise that faster-growing ammonia-oxidizing bacteria establish first, before these comammox Nitrospira gradually take over — a hypothesis consistent with their data but still to be confirmed on a larger sample.
What this study does not show
It covers only three aquariums: the 3- and 8-week figures are an observed order of magnitude, not a guaranteed duration for every tank. Tap water composition, temperature, fish load and the possible use of a commercial starter bacteria product can all shift this timeline noticeably. Nor does the study compare cycling "with fish" against "fishless" cycling: it describes what happens once a tank is filled with water, not the best method to get there.
What this means in practice
The most useful benchmark remains the water test, not the calendar: as long as ammonia or nitrite are measurable, the biofilter is not yet mature, whether at week 2 or week 10. Biofilter beads deserve particular attention since this study identifies them as the substrate where the key nitrifying organisms settle first, a point worth keeping in mind when choosing filter media.
Frequently asked questions
Can fish be added as soon as ammonia drops once?
It is safer to wait for stability confirmed over several consecutive days of testing: this study shows maturation can still progress beyond week 3 in some tanks, up to week 8 for one of the three tracked.
Does a bottled bacteria product really speed up this timeline?
This study does not test such products; it only documents the natural progression of three tanks. Find all our guides on the fishkeeping hub.
See it in the Planète Pets shop: Stick-on digital thermometer for aquarium and terrarium glass (€18.90) · the whole shop
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.
