Knowledge Centre · Fish health
Understand what your fish may be telling you
Gasping, hiding, flashing, clamped fins, an odd swimming pattern — a change in behaviour is your fish's way of showing something's different, not necessarily a sign of disease. This is the browsable route: find the symptom that matches what you're seeing and read the likely causes and safe first checks at your own pace. If something looks wrong right now and you'd rather answer a few quick questions for a ranked starting point instead, use Emergency Diagnosis.
Before you assume illness, check these first
Water quality and environment cause a large share of the signs people search for by symptom name. Ruling these out first takes a few minutes and often changes what you do next:
- Ammonia and nitrite — both are toxic and irritate skin and gills well before more obvious signs appear. See our ammonia and nitrite guides.
- Temperature — confirm your heater and thermometer agree, and that nothing has shifted suddenly.
- Oxygenation — weak surface agitation, overstocking or a heavily planted tank without supplemental aeration can all reduce dissolved oxygen.
- Recent changes — new livestock, décor, feeding changes, a water change, or equipment work can all explain a sudden change in behaviour.
A calm framework for how urgently to act
One symptom on its own rarely confirms one disease — water quality, a recent environmental change, injury and infection can all look similar at first glance, and need to stay distinguishable rather than collapsed into a single guess. This framework is about pace, not diagnosis: it helps you judge how urgently to respond to what you're seeing, then points you toward the guide that matches.
| Level | What this looks like | What to do |
|---|---|---|
| 1. Common isolated behaviour | One fish, occasionally: brief flashing, resting on the bottom, normal caution/hiding, a single missed feeding. | Observe. Note it and keep an eye out for a pattern, but a single, brief, isolated instance rarely needs immediate action on its own. |
| 2. Concerning pattern | One fish, repeated or ongoing: regular flashing or rubbing, sustained appetite loss, clamped fins that don't resolve, a new white spot or two. | Test and monitor. Check ammonia, nitrite, pH and temperature now, and read the matching symptom guide below rather than waiting to see if it clears on its own. |
| 3. Multiple fish or worsening signs | More than one fish showing the same sign, or a single fish's condition visibly worsening over days: spreading white spots, several fish breathing rapidly, fish dying one by one. | Act promptly and investigate. Test water immediately, look for a shared tank-wide cause, and don't wait for it to resolve on its own — see "multiple fish affected" below. |
| 4. Severe or persistent distress | Severe or constant gasping at the surface, a fish that can't stay upright or is unresponsive, rapid deterioration over hours, several unexplained deaths. | Seek experienced or veterinary help. Test and act immediately (see "when to seek urgent help" below), and contact an aquatic veterinarian or a suitably experienced specialist rather than relying on further home troubleshooting alone. |
This framework routes you toward the right depth of response, not a specific cause — the individual symptom guides below (flashing, breathing, gasping, lying on the bottom, clamped fins, appetite loss, white spots, fuzzy growths, swimming upside down, dying one by one) cover the likely causes for each sign specifically.
Browse by symptom
Find the symptom that matches what you're seeing
Grouped loosely by what's most noticeable first. Several of these can share the same underlying cause, so it's worth reading more than one if your fish shows more than one sign.
Breathing problems
Behaviour changes
Fish hiding unusually
Normal caution versus a genuine change worth investigating.
Fish lying on the bottom
Often normal rest, but can signal water-quality stress if it's out of character.
Fish not eating
The common causes of appetite loss and a sensible order to check them.
Fish flashing or rubbing
Scraping against gravel or décor — doesn't automatically mean parasites.
Fish with clamped fins
A general sign of stress or illness, not a specific diagnosis.
Swimming and buoyancy problems
Visible body or skin changes
White spots on aquarium fish
Causes including Ich, look-alike conditions, and what to do.
Marine Ich (saltwater and reef tanks)
A different parasite from freshwater Ich, with different, reef-safe treatment considerations.
Velvet disease
A fine gold or rust dusting, often confused with Ich — freshwater and marine forms are different organisms.
Fish flashing or rubbing
Often noticed alongside skin or gill irritation before any mark is visible.
Fin rot
A whitish, fraying fin edge — usually opportunistic bacteria, not a mystery infection.
White fuzzy growth: fungus or columnaris?
Two different causes that look alike but need completely different treatment.
Dropsy (pinecone scales and swelling)
A sign of serious internal illness, not a single disease — what to check and expect.
Popeye (bulging eye)
One eye or both changes the likely cause — a key first check before anything else.
Multiple fish affected
Fish dying one by one
What gradual, ongoing losses usually point to.
Has my aquarium cycle crashed?
Tell a genuine biological-filter disruption apart from normal variation.
Ammonia in aquariums
The single most common cause behind several fish being affected at once.
Nitrite in aquariums
Test alongside ammonia when more than one fish shows the same sign.
Every symptom guide, A to Z
A complete list if you'd rather scan by name than by category.
- Clamped fins
- Dropsy (pinecone scales and swelling)
- Dying one by one
- Fin rot
- Flashing or rubbing against objects
- Fungus or columnaris (white fuzzy growth)
- Gasping at the surface
- Heavy or rapid breathing
- Hiding unusually
- Lying on the bottom
- Marine Ich (saltwater and reef tanks)
- Not eating
- Popeye (bulging eye)
- Swim bladder problems
- Swimming upside down
- Velvet disease
- White spots
When you need a guided starting point
If you'd rather answer a few questions than browse, Emergency Diagnosis asks what you're seeing, how long it's been happening, and optionally how your water readings compare with normal, then ranks the common, plausible causes and suggests sensible next steps. It's genuinely useful for narrowing things down quickly — it doesn't diagnose a disease, recommend medication, or replace the fuller guides on this page for background reading.
When to seek urgent help
Common questions
Frequently asked questions
Why is my fish acting strangely?
Unusual behaviour can come from water quality, stress, a recent change in the tank, normal biology for that species, or an early sign of illness. Rather than guessing, check ammonia, nitrite and temperature first, then use the symptom that best matches what you're seeing to narrow things down further.
Should I treat my fish before testing the water?
No. Water-quality problems can produce signs that look identical to disease, and medicating first can mask what's actually happening while the underlying cause continues untreated. Test ammonia, nitrite, pH and temperature before reaching for any treatment.
Can poor water quality look like disease?
Yes, very often. Flashing, clamped fins, heavy breathing, hiding and appetite loss can all be caused by ammonia, nitrite, chlorine or a sudden pH or temperature change, with no parasite or infection involved at all. This is exactly why testing comes before assuming a specific illness.
What should I do if several fish are affected at once?
Several fish showing the same sign at the same time points strongly toward a shared, tank-wide cause — most often water quality, oxygenation, or a spreading external parasite — rather than an issue specific to one individual. Test your water immediately and treat it as more urgent than a single affected fish.
How do I know whether a fish problem is urgent?
Severe breathing distress, a fish that can't stay upright or is unresponsive, rapid deterioration over hours, several unexplained deaths, visible serious injury, or a suspected toxin in the tank are all reasons to treat a situation as urgent and act immediately rather than wait and observe.
Related resources
- Aquarium Emergency Diagnosis — a guided tool for prioritising what to check when something looks wrong.
- Water Change Assistant — work out how much water to change and the likely effect on a reading.
- Ammonia in aquariums, nitrite in aquariums and aquarium pH — the core water-chemistry guides behind many symptoms on this page.
- Knowledge Centre — browse every AquaPulse guide by topic.
If this keeps happening
Most single symptoms resolve once the underlying water-quality or husbandry issue is fixed. If a fish in your tank keeps showing signs like this, AquaPulse can help you keep water-test results and tank notes together over time, so a repeating pattern is easier to spot than if you're relying on memory between incidents.
It doesn't diagnose or track symptoms itself — it's a place to keep your own water-test history.