Water Tests and the Nitrogen Cycle: The Essential Kit Before Adding Fish
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
Every year, thousands of fish die within three weeks of purchase, victims of what's known as new tank syndrome. The cause is almost always the same: the aquarium was stocked before the nitrogen cycle was complete. Understanding this mechanism and equipping yourself to measure it is the single best investment of your entire fishkeeping journey.
Contents · 5 sections
The nitrogen cycle in two minutes
Fish waste and uneaten food break down into ammonia, which is highly toxic. Bacteria convert it into nitrites, just as dangerous, then other bacteria convert those nitrites into nitrates, which are relatively harmless at low levels and removed through water changes and plant uptake. These bacterial colonies settle in the filter and substrate over three to six weeks: this is the famous cycling period. During that time, the tank runs filled with water, filter on, without fish.
Test strips or liquid drop tests?
- Test strips (10 to 15 € for 50): readings in one minute, handy for routine checks, but imprecise at low values — precisely the ones that matter for nitrites.
- Liquid drop tests (6 to 12 € per parameter, 30 to 60 € for a complete kit): slower to carry out, but far more reliable. This is the solution we recommend, at least for nitrites and nitrates. JBL, Tetra and other brands offer complete kits with colour charts.
Which parameters to monitor, and when?
During cycling, test nitrites two to three times a week: you'll watch a spike rise and then fall back to zero. As long as nitrites haven't been back at zero for several days, no fish should enter the tank. The readings of a cycle tracked day by day in a lab are set out in our beginner 60-litre aquarium guide. Then, as a routine:
- Nitrates: once a week, to be kept below 25 to 50 mg/l depending on stocking levels.
- pH and hardness (GH/KH): once a month, and before introducing any new species.
- Nitrites: at the slightest doubt (a listless fish, cloudy water, deaths).
The right maintenance habits
The winning trio: change 20 to 30% of the water every week using a water conditioner (5 to 10 €), siphon waste with a gravel vacuum (8 to 15 €), and never wash the filter media under the tap, as explained in our guide to choosing a filter. Measured feeding helps enormously too: revisit our tips for feeding without polluting.
In brief
Patience during cycling, liquid drop tests for nitrites and regular water changes: that's 90% of a successful aquarium right there. Find all the maintenance equipment compared in our care and hygiene section on Planète Pets.
See it in the Planète Pets shop: Hang-on isolation and breeding box for aquariums (€28.90) · 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.
