Buying the fish and the aquarium on the same day: why it is the fatal mistake
No, you should never buy the fish and the aquarium on the same day: a brand-new tank holds no bacteria capable of neutralising the ammonia from waste, and fish poison themselves in it within days. Allow 3 to 4 weeks of cycling between filling the tank and the first inhabitants — it’s the single most important rule in all of fishkeeping.
What happens in a tank filled that very morning?
The water is clear, the temperature right, everything looks ready. But from the first meals, waste releases ammonia, toxic from 0.25 mg/l. In a mature tank, bacteria convert it into nitrites and then into relatively harmless nitrates; in a new tank, those bacteria don’t exist yet. The result: burned gills, fish gasping at the surface, cascading losses in the first fortnight — the great classic among first-aquarium mistakes.
What does a proper cycling look like?
- Day 1: install the substrate and plants, fill with conditioned water, start the filter and heater.
- Weeks 1-2: the filter runs 24/7; a pinch of food every 2 days feeds the cycle.
- Weeks 2-3: the nitrite spike — the critical stage, invisible without tests.
- Week 4: ammonia and nitrites at zero two days running: the tank is ready.
- Stocking: in small groups, one week apart, never all at once.
Can the cycle be sped up safely?
Yes, two honest levers: bottled starter bacteria (8 to 15 €), which save one to two weeks, and above all a handful of filter media taken from a healthy tank (from a friend or an aquarist club), which seeds the tank immediately. No product, however, replaces testing: a liquid NH3/NO2 kit (15 to 25 €) remains essential, like everything touching on water quality.
What if the fish are already in the new tank?
Don’t return them: test every day, change 20 to 30 % of the water as soon as nitrites or ammonia exceed 0.25 mg/l, feed every other day, and add bottled bacteria. It’s a fish-in cycle — stressful but recoverable. And for the ride home from the pet shop, our advice on transporting fish limits the initial stress.
Frequently asked questions
The shop assistant said 48 hours was enough — who should I believe?
The tests: as long as nitrites and ammonia haven’t sat at zero for several days, the tank isn’t ready. 48 hours only serve to stabilise the temperature and off-gas the chlorine.
Plants bought on day one — is that fine?
Yes, excellent even: plant from the moment you fill the tank; plants absorb part of the nitrogen compounds and speed up maturation.
Can a hardy fish be used to cycle the tank?
The sacrificial-fish method is cruel and pointless: a pinch of food or bottled bacteria do the same job without making anyone suffer.
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.