Normal behaviour: the baseline that makes every other observation useful

Normal is species-specific, not general

Much of what reads as alarming in one species is ordinary in another. Some fish rest on the substrate; in others that is a warning. Some are nocturnal and spend the lit period hidden. Some are naturally solitary and territorial, while others are stressed by being kept without a group. Some breathe visibly faster than others simply because of how they are built.

So the useful reference is not a general list of healthy behaviours. It is what each species you keep is documented to do, checked against what your individual animals actually do when the system is stable. Research the species before you buy it, and then spend the first weeks establishing what that animal's ordinary day looks like in your aquarium.

What to build a baseline on

Swimming: where in the water column the animal spends time, whether it holds position or drifts, whether it uses the whole tank or a defended area, and whether its movement is smooth and controlled. Note which fish are usually visible and which are usually hidden.

Respiration: the ordinary rate and depth of gill movement at rest for each species, and where in the aquarium each animal usually sits relative to flow. This is the single most valuable baseline to have, because an increase in respiratory rate is one of the earliest signs of a water-quality problem and it is only detectable against a known normal.

Appetite and feeding behaviour: how quickly each animal responds to food, whether it competes at the surface or waits, and whether it actually gets food. A fish that eats but is outcompeted looks like a fish that is eating.

Appearance: colour and pattern under the same lighting, fin carriage, body condition seen from above as well as from the front, and the condition of eyes, skin, and fins. Photographs taken periodically under consistent light are more reliable than memory.

Social interaction: who chases whom, where the boundaries are, and whether any animal is being kept away from food or shelter. Low-level chasing in territorial species can be ordinary; sustained exclusion is not.

Changes that are ordinary

Not every change is a problem. New arrivals commonly hide, lose colour, and refuse food for a period while they settle. Colour changes with mood, dominance, breeding condition, substrate, and lighting in many species. Activity varies with time of day and with the lighting schedule. Appetite drops when temperature sits at the lower end of a species range.

Breeding behaviour can look like aggression, and in some species it genuinely becomes aggression toward tank mates. Territorial disputes intensify when a layout changes, when an animal reaches maturity, or when the group composition shifts. Recognizing these lets you avoid treating a normal event as a disease.

Changes that deserve attention

A change in respiration is the one to take most seriously, particularly if more than one species shows it at once. So is a fish holding itself abnormally, losing equilibrium, or unable to maintain position. Sudden hiding by a fish that was previously confident, refusal of food by an established eater, rapid darting or rubbing against surfaces, clamped fins held tight to the body, and visible changes in skin, fins, or eyes all warrant a closer look.

When more than one animal changes at the same time, the cause is more likely environmental than infectious. That is the first branch in the decision: test water quality before doing anything else. Ammonia, nitrite, temperature, and — where relevant — salinity and pH cover the majority of environmental causes, and they are cheap to rule out.

Visible signs rarely identify a specific disease. Spots, films, frayed fins, and redness each have several possible causes, and confirming which one usually requires diagnostics that are not available by looking at a tank. The illness signs guide covers this in detail; the important discipline is to describe what you observe accurately rather than to name a disease and then treat for it.

Make observation a habit that produces records

Observation that is not recorded is hard to use. A short daily look — who is out, who is eating, how they are breathing, what the temperature is, whether equipment sounds and flows normally — takes very little time and builds the history that makes a later problem interpretable.

Record events as well as readings: water changes, new additions, feeding changes, treatments, equipment work, layout changes, and power interruptions. When something goes wrong, the question is almost always "what changed recently", and the answer is only available if somebody wrote it down. The maintenance and observation guide covers building this into a routine.

Sources and further reading

Each source is listed with the part of this guide it supports. Source scope matters: a taxonomy record, product label, or general husbandry page does not automatically support every care claim.

• Merck Veterinary Manual: Routine Health Care of Fish
  Daily observation, early detection, and the husbandry record that gives an observation meaning.
• Merck Veterinary Manual: Disorders and Diseases of Fish
  Why visible behavioural signs are rarely specific to one cause, and the role of environment and stress.
• Merck Veterinary Manual: Management of Aquarium Fish
  Water quality as the first thing to check when behaviour changes, and aquarium-management context for stress.