Lights on around the clock, feeding at random hours: the hidden stress behind an overactive goldfish
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
A goldfish swimming nonstop, hugging the glass, is often read as a sign of good health or of boredom needing a new decoration. A study published in 2023 by Saiz, Herrera-Castillo, de Pedro, Delgado, Arvidsson, Marugal-López and Isorna in the journal Animals points to a different cause, tied purely to scheduling (, doi 10.3390/ani13152481). More care guidance is gathered in the fish care and grooming section.
Contents · 5 sections
Two disruptions tested separately
The researchers compared goldfish kept under usual conditions (a regular day/night cycle, meals at a fixed time) with fish subjected either to continuous lighting or to feeding delivered at unpredictable times from one day to the next — a situation not unlike households where tank lighting follows people's comings and goings rather than a timer, and meals depend on the household's own schedule.
Behaviour that looks like anxiety
Fish exposed to continuous light showed higher locomotor activity and more thigmotaxis — the tendency to hug the tank walls rather than explore the centre, a behavioural marker borrowed from anxiety tests used in other vertebrates. Random feeding times produced comparable anxiety-like behaviour. Physiologically, this disruption of the day/night rhythm came with rising cortisol over the course of the experiment, with signs suggesting fatigue of the stress axis as the disruption continued.
What the study does not say
It does not claim that a one-off exception — a night when the living-room light stays on late, a meal served an hour late — is enough to produce this effect: the protocol imposes a repeated, sustained disruption over time. Nor does it compare different lighting durations against each other, unlike what matters for algae growth (see our guide on lighting duration and algae, a separate topic from this one): here, it is the regularity of the cycle and of feeding times that is at issue, not the total duration.
What this changes day to day
The corrective measure tested in the study is simple and requires no purchase: a stable lighting cycle, off at night, and feeding at times kept as regular as possible from one day to the next. A programmable timer already reduces this risk for lighting — see our guide on setting an aquarium lighting timer — but meal regularity mostly depends on the household's own routine, not on equipment.
Frequently asked questions
Does a goldfish that swims a lot necessarily mean stress?
Not necessarily, but the study shows that higher activity combined with wall-hugging can signal a disrupted day/night rhythm rather than simple vitality.
Is leaving the light on overnight occasionally a serious problem?
The study covers sustained, continuous exposure, not a one-off oversight: the documented risk concerns repetition, not the exception. Find all our guides on the fishkeeping hub.
In our shop, for this use: 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.