New fish hugging the bottom: when it stops being acclimation
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
It is the number one question after an introduction, and the usual answer — "give it a few days" — is the one that lets the most situations rot. Our guidance is gathered in the aquarium transport and safety section.
Contents · 6 sections
Before any behavioural explanation: test the water
A fish that stays at the bottom, lethargic and off its food, is first of all showing a clinical sign. Ammonia poisoning presents exactly that way. And those spikes occur during the first two to three weeks of a new tank, before the filter matures — that is, precisely when the first fish go in. Water tests and thermometer first, interpretation afterwards.
How long the novelty response actually lasts
The reflex of staying in the lower half of an unfamiliar tank has long been measured in the zebrafish, a laboratory animal but also a very common beginner's fish. Shinkai, Nakai, Takeshita, Morita and Shimada published in 2025, in Frontiers in Behavioral Neuroscience, a standardisation study of this test (see the study). They propose a three-phase split: the first five minutes as the stress-response period, the 5 to 10 minute window as a transition, and beyond 10 minutes a steady state. At the reference temperature they settle on, 26.5 °C, latency before the first rise into the upper zone was around 108 seconds, with wide individual variation, on very small samples — eight to ten fish per group.
Temperature weighs more than patience
This is the most directly actionable figure in the same work: latency before the first rise went from about 147 seconds at 25 °C to 108 seconds at 26.5 °C, then 47 seconds at 28 °C, with the number of rises increasing in the same direction. Water that is too cool therefore mechanically accentuates bottom-hugging. Check the heater before blaming the fish or its tank mates.
A collaborative study coordinated by Hillman, Fontana and Parker, published in 2025 in Lab Animal, pooled data from 20 laboratories and 488 adult zebrafish (see the study). Two factors stand out and speak to the aquarist: high ambient noise reduced the number of rises, and simply having transported the fish heightened the response, with progressive habituation afterwards. A fish's recent past weighs as much as the tank you put it in.
What these studies do not show
These measurements involve a zebrafish alone, in a bare tank of under a litre, for 5 to 15 minutes. They give no duration applicable to a guppy, a betta or a goldfish introduced into a planted tank with tank mates — décor and the presence of a shoal are precisely what speeds recovery. Nor does generalisation between species hold: Lucon-Xiccato and colleagues showed in 2022, in Scientific Reports, that medaka, a close relative, behave quite differently in this same test. Finally, the test's technical vocabulary does not describe an emotion: it names a measurable response to novelty.
The reading grid to apply
- First minutes: staying on the bottom is the normal reaction, including in a perfectly healthy fish. Do not light the tank, do not feed, do not stand at the glass.
- First hour: watch the trajectory rather than the position — first excursions upwards should appear and grow closer together.
- Beyond 24 to 48 hours with no rises, no feeding and clamped fins: stop putting it down to acclimation. Test the water, check the temperature, and watch the tank mates — night-time aggression is invisible by day.
- Introduce in a group rather than one at a time when the species is gregarious, and take care over the arrival step described in our guide to drip acclimation.
- Use a quarantine tank where possible: the protocol is set out in our article on quarantining new fish.
Frequently asked questions
How long before worrying?
No duration is published for ornamental fish. In practice, one to two days without any return of activity or feeding justifies re-running the water tests rather than waiting.
My fish is breathing fast at the surface: is that the same thing?
No, that is a distinct sign pointing to a lack of oxygen or a poisoning, and it calls for immediate action. Our physiological guidance is in our article on transport stress in fish, and all our guides are on the aquarium hub.
In our shop, for this use: Resin shipwreck hide for aquariums, 4 models (€26.90) · 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.