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Normal behaviour: the baseline that makes every other observation useful

Every illness guide says to watch for changes. That only works if you know what normal looks like in your tank.

2 min readUpdated September 19, 2026

About this guide

Observation

By Fishician

Published September 8, 2026

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The short answer

  • Normal is species-specific. Resting on the bottom is fine for some fish and a warning for others.
  • Breathing is the key baseline. A faster rate is one of the earliest signs of a water problem.
  • Eating isn't the same as getting food. Watch whether each fish actually gets fed.
  • Several fish change at once? Probably the environment. Test the water first.
  • Describe, don't diagnose. Record what you see instead of naming a disease.
An albino bristlenose pleco at the entrance to a breeding cave
A male bristlenose pleco guards its breeding cave. Normal behavior depends on species and context.Photo: Bristlenose Pleco — Anthony Corda · CC BY-SA 3.0. Resized for display.
Ordinary change or worth a closer look?
Usually ordinary
  • New arrivals hiding and pale
  • Colour shifting with mood or breeding
  • Nocturnal fish hiding by day
  • Activity following the light schedule
  • Less appetite at the cool end of the range
Look closer
  • Faster breathing, especially in several species
  • Loss of balance or position
  • A confident fish suddenly hiding
  • An established eater refusing food
  • Rubbing, clamped fins, skin or eye changes

Build a baseline on

  • Swimming: where each fish spends time, and whether it holds position smoothly.
  • Breathing: the resting gill rate for each species and where it sits relative to flow.
  • Feeding: how fast each fish responds and whether it gets food.
  • Appearance: colour, fins and body condition, photographed under the same light.
  • Social: who chases whom and where the boundaries are.

Behaviour that looks like trouble but may not be

  • Breeding can look like aggression, and sometimes becomes real aggression toward tank mates.
  • Territory disputes flare after a layout change, when a fish matures, or when the group changes.
  • Low-level chasing in territorial species can be normal; sustained exclusion from food or shelter is not.

Make it a daily habit

  • A short daily look: who's out, who's eating, how they're breathing, temperature, equipment noise and flow.
  • Record events too: water changes, additions, feeding changes, treatments and power cuts.
  • See maintenance and observation and warning signs.

Build the baseline early

  • Research what each species is documented to do before you buy it.
  • Spend the first weeks noting each fish's ordinary day in your tank, when the system is stable.
  • Note which fish are usually visible and which usually hide.
  • Photograph them under the same light every so often; memory drifts.

Sources

Each source is listed with the part of this guide it supports.