Something is off with your fish and you can feel it. They're not eating. They're hiding. They're swimming strangely. Your gut says something is wrong, and your gut is probably right.

The good news is that fish rarely behave oddly for no reason. Unusual behaviour is almost always a signal, and once you learn to read the signals, you can usually identify the problem and fix it before it becomes a serious loss.

This guide walks through the most common behavioural changes hobbyists notice, what each one usually means, and what to do about it. Work through it like a checklist and you'll have your answer most of the time.

The First Rule: Check the Water Before Anything Else

Before you jump to any conclusion about disease, aggression, or equipment failure, test your water. 95% of all issues are usually borne from poor water quality of one or multiple parameters

The majority of "weird fish behaviour" cases trace back directly to water quality. Ammonia, nitrite, or nitrate spikes are the most common culprit. Temperature swings or changes in pH are the second most common. Both can produce symptoms that look exactly like disease but clear up completely once the water is fixed.

Run your test kit before you do anything else. Check: ammonia, nitrite, nitrate, temperature, and pH. If you don't have a liquid test kit, pick up an Aquasonic or API Master Test Kit. Strip tests exist but liquid tests are far more reliable and worth the investment, particularly when trying to identify and diagnose an issue..

If ammonia or nitrite reads anything above 0 ppm, do a 25% water change immediately. That single action can stop a problem from escalating while you figure out the root cause.

With that covered, here's how to read what your fish is trying to tell you.

Behaviour 1: Gasping at the Surface

What it looks like: Fish congregating at the very top of the tank, mouths breaking the surface, appearing to gulp air.

What it usually means: Not enough oxygen in the water or a problem with their gills. This is an urgent symptom.

There are a few reasons dissolved oxygen can drop:

• Poor surface agitation. Oxygen enters the water at the surface. A still, glassy surface with no movement is a warning sign, especially in warm weather when oxygen dissolves less efficiently in warmer water.
• Overstocked tank. Too many fish produce too much waste and consume too much oxygen.
• Ammonia or gill damage. High ammonia physically burns gill tissue, making it harder for fish to extract oxygen even when it's available. This is why gasping and ammonia spikes often go together.
• Heat. In Australian summers, unheated tanks in warm rooms can climb to 30°C or above. Warm water holds far less dissolved oxygen than cool water.

What to do: First, increase surface agitation immediately. Point your filter outlet at the surface, add an airstone, or run a fan across the water surface to cool it and boost gas exchange. Then test for ammonia. If the tank is overstocked, that's a longer-term problem to solve.

Behaviour 2: Hiding and Not Coming Out

What it looks like: A fish that was previously active and visible now spends most of its time wedged behind a decoration, in a corner, or under a plant. Won't come out even at feeding time.

What it usually means: Stress from one of a few sources.

• New tank stress. Newly introduced fish often hide for 1-2 weeks while they adjust to their environment. This is normal. As long as they're eating (even tentatively) and have no visible symptoms, wait it out.
• Bullying. Check who else is in the tank. If another fish is chasing or nipping the hiding fish, the hider is being pushed out of its territory. Watch the tank closely for 10-15 minutes without disturbing it and you'll usually catch the aggressor.
• Illness. A fish that hides, stops eating, and has visible physical changes (white spots, frayed fins, bloating, pale colouring) is likely sick. Keep reading the list below and check the symptoms section.
• Water quality. A fish retreating to a corner and barely moving is often reacting to ammonia or nitrite.

What to do: Test the water, observe the other fish for aggression, and check for any physical symptoms on the hiding fish itself.

Behaviour 3: Erratic or Unusual Swimming

This is a broad category, but the type of erratic movement usually points to a specific problem.

Darting around frantically, crashing into glass

Usually a reaction to something that just changed in the tank. Common triggers include a water change that altered temperature or chemistry suddenly, new tank mates being added, or a loud noise near the tank. If it settles within an hour or two, the fish was startled rather than sick.

If it continues for hours or spreads to multiple fish, check ammonia and nitrite immediately.

Spinning or swimming in tight circles

Often a sign of a neurological issue or severe parasitic infection (particularly flukes affecting the inner ear or brain). Can also appear in fish that have suffered physical injury from being chased. A fish spinning without stopping is in serious distress and usually difficult to save, but test the water first regardless.

Floating or sinking involuntarily, swimming upside down or sideways

This points strongly to swim bladder problems. The swim bladder is an internal gas-filled organ that controls buoyancy. When it malfunctions, fish lose control of their position in the water.

Swim bladder issues are common in goldfish and bettas. Common causes include overfeeding (constipation puts pressure on the swim bladder), eating dry pellets too quickly (they expand after swallowing), or bacterial infection. They can also be a symptom of other underlying problems.

The simplest first response is a 24-48 hour fast. Skip feeding completely. For goldfish especially, constipation is a frequent cause and a short fasting event often resolves it. If the issue persists after 48 hours, a bacterial infection is more likely.

Scraping or rubbing against decorations and substrate (flashing)

This is called "flashing" and almost always means the fish has a parasite irritating its skin or gills. White spot (Ich) is the most common cause, but gill flukes and other external parasites produce the same behaviour.

Look carefully at the skin under good lighting. White spot looks like the fish has been dusted with fine salt or sand. Gill flukes won't be visible but the fish will be scratching and may show rapid gill movement or laboured breathing.

Behaviour 4: Not Eating

What it looks like: Fish ignores food completely, or takes it into its mouth and spits it back out.

What it usually means: This one is tricky because several different things cause it.

• Just arrived. Newly introduced fish commonly refuse food for 3-7 days. Don't panic. Keep offering food in small amounts and remove what they don't eat after a few minutes.
• Wrong food. Different fish have different feeding preferences. Carnivores offered flake food, or bottom dwellers being fed floating pellets, will refuse to eat. Make sure the food type matches the fish.
• Water temperature too low. Fish are cold-blooded. Their metabolism slows as temperature drops, and appetite drops with it. A fish in water that's 2-3°C below its preferred range will eat poorly or not at all. Check the thermometer.
• Stress or illness. Loss of appetite almost always accompanies the early stages of any health problem. If a fish that has been eating fine suddenly stops and also shows other symptoms, treat it as a health issue and investigate further.
• Overcrowding or bullying. A fish being chased away from the food by dominant tank mates will appear not to eat when it's actually just not getting the chance. Watch feeding time closely.

Behaviour 5: Pale or Faded Colours

What it looks like: A fish that was vivid and colourful now looks washed out, grey, or dull.

What it usually means: Stress is almost always the cause. Fish that are stressed, threatened, or unwell dull their colours. It's a physiological response.

Common triggers: poor water quality, wrong tank mates, temperature outside their preferred range, insufficient hiding spots (some fish need breaks from being visible), or the early stages of disease.

A fish that has faded noticeably and hasn't recovered its colour within a day or two needs the water tested and the tank setup reviewed. Once the stressor is removed, colours usually return within a few days.

Behaviour 6: Clamped Fins

What it looks like: Instead of holding their fins open and extended, the fish keeps them pressed tight against its body.

What it usually means: Clamped fins are a classic stress and illness signal. The fish is uncomfortable.

Very common causes include poor water quality, a temperature drop, the early stages of a parasitic or bacterial infection, and general stress from incompatible tank mates. Clamped fins alone don't tell you which specific problem you're dealing with, but they tell you clearly that something is wrong.

Test the water, check the temperature, and look for any other symptoms. Clamped fins with white spots points to Ich. Clamped fins with frayed edges points to fin rot. Clamped fins with no other visible symptoms and clean water parameters often resolves once temperature is corrected.

The Most Common Diseases & What They Look Like

Once you've ruled out water quality as the cause, here's how to identify the most common diseases by their visible symptoms.

White Spot (Ich / Ichthyophthirius multifiliis)

Symptoms: Fine white spots covering the body and fins, resembling salt or sand grains. Often accompanied by flashing, clamped fins, rapid breathing, and lethargy.

Cause: A parasitic protozoan. Almost always introduced via new fish, live plants, or shared equipment. Stress and temperature fluctuations trigger outbreaks in fish already carrying the parasite.

What to do: Treat the whole tank, not just the affected fish, because by the time you see spots the parasite is almost certainly throughout the water column. Raising the temperature slowly to 28-29°C speeds up the parasite's life cycle and makes treatment more effective. Use a commercial Ich treatment, such as Aquasonics ‘Ichonex’ and complete the full course.

Fin Rot

Symptoms: Fins look ragged, frayed, or as if the edges are being eaten away. In advanced cases the fin may be partially gone. The affected edges often have a whitish or red-tinged appearance.

Cause: Most commonly a bacterial infection. Almost always triggered by poor water quality, physical injury from fin-nipping tank mates, or stress that weakens the immune system.

What to do: Improve water quality first as it's both the treatment and prevention. Do a water change, check parameters, and look for fin nippers in the tank. A broad-spectrum antibacterial treatment, such as Aquasonics Bio-Tet or Bactonex helps in moderate to severe cases.

Swim Bladder Disorder

Symptoms: Fish floats at the surface involuntarily, sinks to the bottom, or swims at an angle. May roll or appear unable to right itself.

Cause: Overfeeding, constipation, eating dry food too fast, or bacterial infection. Fancy goldfish and bettas are most commonly affected.

What to do: Fast the fish for 24-48 hours. If constipation is the cause, this usually resolves it. Feed frozen peas (shell removed, just the soft inner) for a day or two afterward as a gentle laxative. If no improvement after 48-72 hours, a bacterial infection is the more likely cause and may require antibiotic treatment.

Dropsy

Symptoms: A severely bloated, swollen belly with scales that stick out from the body like a pinecone. By the time pineconing scales are visible, the disease is advanced.

Cause: A bacterial infection of the kidneys causing fluid to accumulate. Often a secondary infection in a fish already weakened by stress or poor water quality.

What to do: Dropsy at the pineconing stage has a very poor prognosis. Antibiotic treatment in a separate hospital tank can sometimes help in early cases. The most important response is to prevent it through good water quality and low-stress tank conditions.

Velvet

Symptoms: A fine gold or rust-coloured dust on the body, most visible on dark fish under bright or angled light. Fish often clamp fins and scratch. Can look like Ich but finer and with a yellowish or golden tint.

Cause: A different parasitic protozoan to Ich. Highly contagious and can progress quickly, especially in weakened fish.

What to do: Treat immediately with a velvet-specific treatment, such as Aquasonics Ichonex (freshwater), Oodonex (marine) or Bactonex. Dimming lights can slow the parasite's reproduction as it's photosynthetic. Treat the whole tank.

A Quick Diagnosis Guide

When to Set Up a Hospital Tank

A hospital tank is simply a spare tank (even a 20-litre bucket or container will do in an emergency) with a filter, a heater, and clean water. It serves two purposes: it lets you treat a sick fish with medication without affecting the main tank as some medications can also harm the beneficial bacteria in a filter, and it removes the sick fish from any stress caused by tank mates.

You don't need a permanent second setup. A small spare tank kept empty and dry can be filled and running within 30 minutes when needed. It's worth having.Make ure the water quality parameters are within the fishes range. This means you may need a spare heater as well for the hospital tank.

The Underlying Pattern

Almost every case of fish acting oddly comes back to one of three root causes: water quality, tank mates, or disease. Work through them in that order and you'll find the answer most of the time. Remember, that 95% of diseases stem from an initial water quality problem.

Test before you medicate. Do a water change before you add anything to the tank. Remove the stressor before you expect the fish to recover. These three steps, done in the right sequence, resolve the vast majority of problems hobbyists face.

If you're unsure what you're dealing with or need help identifying the right treatment for your tank, the team at LiveFish is here to help. We'd rather you contact us early than wait until it's too late.

Please note: The information in this article is intended as general guidance for aquarium hobbyists. Fish often display similar symptoms across multiple issues/diseases, and aside from some common ailments, accurate diagnosis requires more than a visual external observation, although these observations certainly hope with a possible diagnosis. We are not veterinarians. If you are dealing with a serious or ongoing illness in your fish, we recommend consulting a qualified aquatic veterinarian.