Setting up a freshwater aquarium involves more than filling a glass box with water and dropping fish in. Before any fish arrive, the tank needs to develop a colony of nitrifying bacteria capable of processing the ammonia those fish will continuously produce. Without this biological filter in place, ammonia levels rise fast enough to cause gill damage or death within days. The process of building this bacterial colony is called cycling.
What actually happens during cycling
Ammonia enters the water from fish respiration, uneaten food, and decomposing organic matter. Two genera of bacteria handle the conversion chain. Nitrosomonas species oxidise ammonia (NH₃) into nitrite (NO₂⁻). Nitrospira species then oxidise nitrite into nitrate (NO₃⁻). Nitrate accumulates harmlessly at low concentrations and is removed through water changes.
The critical point is that these bacteria colonise filter media — primarily the sponge, ceramic rings, or bio-balls inside your filter — not the water column itself. A large, well-populated filter can process significant ammonia loads. A filter that has been cleaned too aggressively or run dry even briefly can lose most of its bacterial population.
Why Polish tap water matters here
Warsaw and most major Polish cities add chloramine to drinking water, not just chlorine. Chloramine does not off-gas on standing — it requires a dechlorinator specifically rated for chloramine (such as Seachem Prime or Tetra AquaSafe Plus). Using a basic dechlorinator will leave chloramine in the water, which inhibits bacterial colonisation and stresses fish.
Fishless cycling: the recommended approach
Fishless cycling avoids exposing any animals to the ammonia and nitrite spikes that occur during the process. You add an ammonia source to the empty, dechlorinated tank, let bacteria establish, and then confirm with test kits before adding fish.
What you need
- A liquid test kit measuring ammonia, nitrite, and nitrate (API Master Test Kit is widely available in Polish aquarium shops)
- A pure ammonia source — either pure ammonia solution (without surfactants or fragrance) or dried fish food as a decomposing ammonia source
- A dechlorinator rated for chloramine
- A running filter with media, a heater set to 26–28°C to speed bacterial growth, and a light source on a timer
Step-by-step process
- Fill the tank with tap water and add dechlorinator at the recommended dose.
- Run the filter and heater. Set temperature to 26–28°C — higher temperatures accelerate bacterial colonisation.
- Add ammonia to reach 2 ppm. If using pure ammonia solution, roughly 1 ml per 40 litres typically hits this level, but test after each addition.
- Test ammonia and nitrite every two days. Do not add more ammonia until levels drop.
- After 1–2 weeks, nitrite should start rising. This confirms Nitrosomonas are active.
- Once nitrite peaks and then drops to near zero, test for nitrate. A readable nitrate level with zero ammonia and zero nitrite means the cycle is complete.
- Do a 50–80% water change to remove accumulated nitrate before adding fish.
The entire process typically takes 3 to 6 weeks at 26°C in a new tank with no bacterial seed material. Using filter media, gravel, or water from an established tank can cut this to 1–2 weeks.
Fish-in cycling: when it cannot be avoided
If fish are already in the tank — perhaps bought before the owner knew about cycling — the approach changes. The goal becomes keeping ammonia and nitrite below toxic thresholds while bacteria establish naturally.
| Parameter | Safe range (fish-in cycle) | Immediate action level |
|---|---|---|
| Ammonia (NH₃/NH₄⁺) | Below 0.5 ppm | Above 1 ppm — emergency water change |
| Nitrite (NO₂⁻) | Below 0.5 ppm | Above 1 ppm — add aquarium salt to block uptake |
| Nitrate (NO₃⁻) | Below 40 ppm | Above 80 ppm — 30–40% water change |
During fish-in cycling, test daily and perform 20–30% water changes whenever ammonia or nitrite climbs above 0.5 ppm. Feed sparingly — once every two days rather than daily — to reduce the organic load. Avoid cleaning the filter during this period.
Aquarium salt as a nitrite buffer
During the nitrite spike phase, sodium chloride (aquarium salt, not table salt) at 1 gram per litre reduces nitrite toxicity by competing with the same uptake pathway fish use to absorb nitrite through their gills. This does not reduce nitrite in the water — it only limits how much fish absorb. Do not use salt in tanks with scaleless fish (corydoras, loaches) or plants sensitive to salinity.
How to confirm the cycle is complete
A tank is considered cycled when it passes the "ammonia challenge" test: add ammonia to reach 2 ppm, then test again after 24 hours. If both ammonia and nitrite read zero (or near zero) at the 24-hour mark, the bacterial colony is large enough to process typical fish loads. Nitrate should have risen slightly.
Maintaining a cycled filter
Once the cycle is established, the filter media requires careful handling. Rinsing media in tap water kills bacteria. Instead, rinse sponge media in a bucket of water removed from the tank during a water change. Replace only part of the media at a time — never all of it at once — to preserve bacterial populations. Avoid running the filter dry for more than a few minutes.
Related reading: Choosing aquarium plants for beginners — plants absorb nitrate and contribute to tank stability after cycling.
