The First 30 Days of a New Aquarium

A week-by-week guide to the first month of a new aquarium: what to test, which changes are normal, when the first fish can go in and the mistakes sources warn about.

The first month of a new aquarium is mainly about growing nitrifying bacteria in the filter: test ammonia, nitrite and pH regularly, and add the first fish only once ammonia and nitrite both read zero. Thirty days may not be enough, because extension and hobby sources put the establishment of a biofilter at roughly four to eight weeks.

Why the first month is about bacteria

Fish excrete ammonia; ammonia-oxidising bacteria convert it to nitrite, and nitrite-oxidising bacteria convert nitrite to the less toxic nitrate. In tanks and aquaria these bacteria colonise the biological filter, and UF/IFAS states that a new biofilter needs six to eight weeks to build up enough bacteria to control ammonia and nitrite. Seriously Fish gives four to six weeks as normal for a fishless cycle, sometimes faster when a bacterial source is added at the start and occasionally up to eight weeks; Wikipedia notes reported cases of up to six months. The chemistry itself is covered in the New Tank Syndrome guide, and the ammonia-dosing method in the Fishless Cycling guide.

Week 1: fill, run and observe

Run the filter and heater and confirm that all equipment works before any livestock is considered; hobby guidance recommends a settling period for this.
Keep the filter outlet moving the surface. Nitrifying bacteria need oxygen and alkalinity, and UF/IFAS reports that common nitrifiers do not work effectively below a total alkalinity of 20 mg/L.
Start a written log of ammonia (total ammonia nitrogen, TAN), nitrite, pH and temperature. UF/IFAS explains that the toxic un-ionised share of TAN can only be estimated when pH and temperature are known.
Milky water that appears overnight is usually a bacterial bloom. Practical Fishkeeping describes it as a harmless side effect of the tank establishing itself biologically that usually clears within a few days.

Weeks 2–3: ammonia and nitrite peaks, brown film

During a cycle ammonia falls first and nitrite then rises sharply before it, too, drops. A 2025 study that followed three newly set-up home aquariums with fish found that nitrite peaked in week 2 in two of them and fell below detection the following week, while in the third aquarium ammonia stayed high from week 2 to week 7 and only cleared in week 8. All three used the same municipal tap water, so timing varies between tanks and cannot be predicted from the calendar alone.

A brown, dusty coating on glass, decor and plants is typical of diatoms, single-celled algae whose cell walls are built from silica. Hobby plant sources describe their appearance in the first weeks of a new tank as a normal stage that is later crowded out by green algae; the coating can be siphoned or wiped off.

Weeks 3–4 and beyond: deciding the tank is ready

Seriously Fish treats a cycle as complete when ammonia and nitrite both read zero for more than a couple of days in a row; nitrate, the end product, keeps rising.
After a fishless cycle Seriously Fish advises a large water change of 80–90 % to remove accumulated nitrate before fish go in.
Fish are then added in small numbers over a period of weeks; Wikipedia describes increasing the population over 6 to 8 weeks while monitoring water quality. The Stocking Order guide and the Acclimating Fish guide cover this step.
If ammonia or nitrite are still detectable at day 30, the cycle is not finished and the wait continues.

Testing during the first month

In a healthy tank ammonia should always be zero; UF/IFAS treats any measurable ammonia as a sign that the system is out of balance.
UF/IFAS reports nitrite as toxic to fish at levels as low as 0.10 mg/L.
UF/IFAS recommends testing ammonia at least weekly in any system, more often whenever ammonia is found, and immediately whenever fish change in appearance or behaviour; records reveal trends that a single test cannot.
During a fishless cycle Seriously Fish tests daily and checks pH every few days, because the bacteria go dormant if pH falls too far.
Ammonia test kits based on the Nessler method can give falsely high readings after water conditioners or ammonia-binding products are used; the salicylate method is not affected (UF/IFAS).

Mistakes sources flag in the first month

Adding too many fish too quickly. Wikipedia lists the resulting ammonia stress as loss of movement and appetite, red and inflamed gills, fins and body, and sometimes gasping at the surface.
Overfeeding, which UF/IFAS names as a major cause of high ammonia; uneaten food only adds to the load.
Large water changes to clear a bacterial bloom in a tank without fish: Practical Fishkeeping warns that they can slow maturation.
Letting alkalinity run out, which UF/IFAS links to sudden pH drops that can kill both fish and nitrifying bacteria.
Using medicines, especially antibiotics, in a young tank: UF/IFAS notes they can harm nitrifying bacteria, so ammonia and nitrite should be tested more often after any treatment.