Most “sick fish tank” problems start the same way: a fish that won’t eat, hides more than usual, or shows new spots, frayed fins, or rapid breathing. The risk isn’t just losing one fish — once water quality slips or an infection gets a foothold, the whole aquarium can follow.
The quickest path to a reliable answer is to separate “environment problem” from “infectious problem”. Start with water tests, then observe the fish closely, and only then reach for medications. Many treatments fail simply because the tank’s chemistry is still hurting the fish.
First, check the tank (most illness begins here)
In home aquariums, poor water quality is one of the most common underlying causes of fish stress and deaths, especially in newer tanks where the biological filter isn’t fully established. When the nitrogen cycle isn’t stable, toxic ammonia and nitrite can rise quickly even if the water looks clear.1, 2
Do these checks immediately
- Test ammonia and nitrite. In a healthy, cycled tank they should be 0. Ammonia is toxic even at low levels, and becomes more toxic at higher pH and temperature.2
- Test nitrate. Nitrate is less toxic than ammonia/nitrite, but rising nitrate tells you waste is building up and water changes are falling behind.2
- Check temperature stability. Sudden swings can weaken fish and make outbreaks more likely.3
- Watch oxygenation and surface behaviour. Fish “piping” at the surface can point to low dissolved oxygen or nitrite problems, even when filtration is running.3
If ammonia or nitrite is above zero
Treat it as an emergency, even if only one fish looks unwell. Do a partial water change, improve aeration, reduce feeding for a day or two, and re-test. Keep changes steady rather than dramatic, especially with temperature and pH. Rinse filter media gently in old tank water — not tap water — so you don’t strip out beneficial bacteria.2
Common signs a fish is unwell
Physical signs
Many conditions look similar early on, so focus on patterns rather than a single mark.
- White spots or fine “dusting” (can suggest parasites such as ich/velvet, but confirm with other signs)
- Frayed, clamped, or rotting fins (often linked with stress and poor water, sometimes with infection)
- Cloudy eyes, excess mucus, inflamed gills (can appear with irritants like chlorine or ammonia, or infection)3
- Bloating or raised scales (a sign of internal trouble; prognosis can be poor)
Behaviour changes that matter
- Hiding, hovering, or sitting on the bottom for long periods
- Rapid gill movement or hanging at the surface
- Flashing (rubbing on décor or gravel), which can indicate irritation from water chemistry or parasites
- Sudden aggression or being relentlessly harassed by tank mates
Why fish get sick: the main culprits
1) Water quality and “new tank syndrome”
Ammonia and nitrite are direct toxins. Ammonia can damage gills and skin and can trigger abnormal swimming; nitrite interferes with oxygen transport in the blood (“brown blood disease”).3
In new tanks, the biofilter typically takes weeks to mature. During this window, frequent testing and smaller, regular water changes reduce risk.2
2) Infectious disease (bacterial, fungal, viral)
Opportunistic infections often follow stress. The fish may show fin erosion, ulcers, fuzzy growths, or cloudy eyes. Good hygiene and stable water reduce the chances of these organisms gaining ground.1
3) Parasites
External parasites can cause flashing, clamped fins, breathing changes, and visible spots or a dusty sheen. Parasites may be present at low levels without obvious harm until fish are stressed by transport, crowding, or deteriorating water.4
Diagnosing a sick fish without guessing
Step-by-step approach
- Step 1: Test the water first. Fixing water often improves symptoms within 24–72 hours, and makes any later treatment safer.2
- Step 2: Observe for a full day. Note breathing rate, appetite, faeces, body marks, fin position, and whether multiple fish are affected.
- Step 3: Check for triggers. New fish added recently, missed water changes, a blocked filter, overfeeding, a heater failure, or a recent chemical exposure (cleaners, aerosols).
- Step 4: If possible, move the fish to a hospital tank. It reduces spread and lets you treat accurately without medicating the whole display.
Useful tools
- Liquid water test kit (ammonia, nitrite, nitrate, pH)
- Thermometer (stable temperature matters as much as the number)
- Spare tub or small tank for a basic hospital/quarantine set-up
Treatment options (and when to use them)
Start with environmental treatment
Before medications, correct water quality, increase oxygenation, and reduce stressors (overcrowding, bullying, sudden light changes). Supportive care is not “doing nothing” — it often removes the actual cause.
Medications: use with care
Over-the-counter fish medications can help, but they work best when you’re treating a specific, likely diagnosis (for example, an external parasite treatment for a clear parasite pattern). Remove activated carbon during treatment so it doesn’t adsorb the medication. Follow label directions exactly.
Be cautious with copper-based treatments: copper is toxic to many invertebrates and can affect biofilters, so ammonia and nitrite may rise during and after treatment. Continued monitoring is essential.1
“Natural remedies”
Aquarium salt is sometimes used as a short-term supportive measure for certain freshwater problems, but it is not universally safe (many plants and some species are sensitive), and it won’t solve ammonia, nitrite, or an advanced infection. Use it only when you understand the species’ tolerance and the reason you’re using it.
When to consult an aquatic veterinarian
Seek professional help if fish are dying quickly, if symptoms persist after water quality is corrected, or if you see ulcers, severe bloating, persistent inability to swim normally, or repeated outbreaks. A vet can help with diagnosis (and, in some cases, targeted prescriptions) rather than trial-and-error.
Prevention: keeping illness rare
Regular maintenance that actually prevents disease
Healthy aquariums stay steady. Effective filtration and routine partial water changes keep waste products down and oxygen levels up, which supports both fish welfare and the biofilter that protects them.2, 5
- Do small, regular water changes (often 10–25% weekly is recommended for many home tanks, adjusted for stocking and feeding).5
- Vacuum the substrate to lift uneaten food and faeces before it decays.5
- Don’t “deep clean” the filter in tap water; rinse media in removed tank water to protect beneficial bacteria.2
Quarantine new fish
New arrivals are a classic way disease enters an established tank. Quarantine gives fish time to recover from transport stress and lets you watch for parasites or infections before they reach your display. A quarantine period of a few weeks is commonly recommended in fish health guidance for reducing spread between groups.6
Don’t release aquarium fish, plants, or water
Never dump aquarium contents into creeks, drains, or waterways. Apart from the animal welfare risk, it can spread pests and disease into the environment. Dispose of unwanted fish and aquarium water safely, following local biosecurity guidance.7
Feeding and stress: the quiet drivers of health
Balanced diet, not constant food
Offer a diet suited to the species (herbivore, carnivore, omnivore), and feed modest amounts that are eaten promptly. Overfeeding doesn’t just make fish overweight — it rots into ammonia and loads the biofilter.
Keep the social mix calm
Stress rises when fish are crowded, chased, or kept with incompatible tank mates. Provide cover (plants, rockwork, caves) so shy fish can avoid constant contact, and keep lighting predictable. In aquariums, calm behaviour is usually a sign the environment is stable.
Final thoughts
When a fish looks unwell, the tank itself is the first place to look. Water tests, steady corrections, and careful observation usually reveal whether you’re dealing with toxins, stress, or a contagious disease. Treat the cause, not the label on a bottle, and the aquarium often settles back into its quiet, working rhythm.
References
- Merck Veterinary Manual — Management of Aquarium Fish
- RSPCA Knowledgebase — Why is water quality important when setting up a fish aquarium?
- Merck Veterinary Manual — Table: Common Environmental Hazards for Fish
- Tetra — Fish illnesses: how to spot them
- RSPCA Knowledgebase — How should I care for my tropical fish?
- University of Florida IFAS Extension — Fish Health Management Considerations in Recirculating Aquaculture Systems (Part 1)
- Northern Territory Government — Aquatic biosecurity: how to dispose of aquarium contents
Veterinary Advisor, Veterinarian London Area, United Kingdom