How to Tell If Your Fish Is Sick | Symptoms & Diagnosis Guide

Your fish cannot tell you when they feel unwell, so you have to learn to read the signs. This beginner-friendly guide shows you how to tell if your fish is sick, how to diagnose the cause, and what to do next. You will learn the difference between normal behavior and red flags, how to rule out water problems, and how to set up simple care at home. Keep this page handy and move through it step by step whenever you notice something “off.”

What Healthy Fish Look Like

Normal Behavior and Posture

Healthy fish swim smoothly and confidently. They keep their fins open, not clamped tight to the body. They balance well without drifting, tilting, or rolling. They interact with tank mates without hiding all day or darting in panic. Resting is normal, but a healthy fish still reacts quickly to food or movement near the glass.

Appetite and Waste

A healthy fish notices food and eats eagerly. Sudden loss of appetite is a common early sign of stress or illness. Waste should be firm and sink soon; very long, white, or clear stringy poop may point to internal problems, poor diet, or parasites.

Breathing and Gills

Watch the gill movement. Normal breathing is steady and not too fast. Gills should be pink to red inside without visible swelling. Gasping at the surface or breathing rapidly while resting can mean low oxygen, gill parasites, or poor water quality.

Skin, Scales, Fins, and Eyes

Skin should be smooth without white grains, gold dust, or cottony tufts. Scales lie flat. Fins are clear, not frayed, red, or melted at the edges. Eyes are clear and not cloudy or popping out. Slight color changes after lights turn on or off can be normal, but persistent pale or dark stress coloration can signal trouble.

Water Test Baseline

Healthy fish live in stable water. Keep a liquid test kit and know your normal readings: ammonia 0 ppm, nitrite 0 ppm, nitrate under 20–40 ppm (lower is better), temperature appropriate for the species, and a stable pH. Many “sick fish” are actually reacting to water issues.

Early Warning Signs You Should Never Ignore

Sudden Behavior Changes

If your fish hides all day, stops greeting you at feeding time, or darts and crashes into objects, that is a warning. Consistent behavior change across several fish at once is a strong clue that water quality is the problem.

Surface Gasping or Clamped Fins

Frequent trips to the surface for air, hanging near the filter outflow, or fins pressed tight to the body signal stress. Clamped fins often appear early during parasite infections, chilling, or poor water conditions.

White Dots, Dusting, or Fuzzy Patches

White salt-like spots usually point to ich. A fine velvety gold or rust dust suggests velvet. Fuzzy cotton-like tufts usually indicate fungus or a bacterial lesion. These are visible signs that help you narrow down the cause.

Red Streaks, Ulcers, or Fin Erosion

Red lines in fins, raw patches, and eroding edges often mean bacterial infection or ammonia burns. Left untreated, these can quickly worsen.

Bloating, Pineconing, or Stringy Poop

Swelling with raised scales that look like a pinecone is a serious sign of organ failure or fluid retention often called dropsy. Long, white, thread-like feces can point to internal parasites or irritation.

Scratching and Flashing

Rubbing against rocks, sand, or decorations is called flashing. Occasional scratching can be normal, but frequent flashing often points to skin or gill parasites or an irritant in the water.

Step-by-Step Diagnosis Checklist

Step 1: Observe From a Distance

Stand back for a minute. Fish act differently if you are too close. Note swimming pattern, breathing rate, fin position, appetite, and interactions. Compare with the fish’s normal day-to-day behavior.

Step 2: Test the Water Right Away

Test ammonia, nitrite, nitrate, pH, and temperature. If available, also check KH and GH for stability. Any amount of ammonia or nitrite is dangerous. High nitrate and big pH swings stress fish. Record the numbers and the time of day.

Step 3: Match Symptoms to Likely Causes

Rapid breathing and surface gasping with fish across the tank point to low oxygen, high ammonia, or high nitrite. White grains on the body point to ich. Gold dust points to velvet. Frayed fins with red edges suggest fin rot or ammonia burn. Cottony tufts suggest fungus or bacterial lesions. Bloating with raised scales suggests dropsy. Stringy white feces and weight loss point to internal parasites.

Step 4: Review Recent Changes

Think about what changed in the past 7–14 days. New fish or plants, skipped water changes, a new filter, overfeeding, a deep gravel clean, or a power outage all matter. Disease outbreaks often follow stress or a new introduction.

Step 5: Isolate the Affected Fish

Move the sick fish to a hospital tank if possible. This reduces stress, lets you treat accurately, and protects the main tank’s biofilter from medications. If the whole tank is affected, treat the display after confirming the cause.

Step 6: Decide: Environmental or Infectious

Bad water readings, multiple fish affected at once, or instant onset after maintenance usually mean environmental stress. Treat the water first. Spotty, patchy lesions, flashing, or gradual spread fish to fish suggest parasites, bacteria, or fungus.

Step 7: Start Supportive Care

Improve oxygen with extra aeration, keep the tank dark and quiet, and raise the temperature only if you know it helps and your species tolerates it. Add aquarium salt cautiously if your fish species can handle it. Make small, frequent water changes to reduce toxins.

Step 8: Reassess After 24–48 Hours

Retest the water and recheck symptoms. If signs improve after water fixes, continue supportive care. If not, choose a targeted medication based on the most likely diagnosis and follow the label exactly.

Common Problems and What Their Symptoms Look Like

Ich (White Spot Disease)

Look for small white dots like salt, usually starting on fins and spreading to the body. Fish may scratch, clamp fins, and breathe faster. Ich spreads quickly. Treatment often involves raising temperature within safe limits for the species, adding salt for tolerant fish, and using a proven ich medication. Continue treatment through the parasite’s life cycle, even after spots fall off.

Velvet (Gold Dust Disease)

A fine, dusty, gold or rust film under strong light suggests velvet. Fish often clamp fins, breathe hard, and avoid light. Prompt treatment with a specific anti-parasite medication and light reduction is important. Velvet can be severe if missed early.

Fin Rot

Edges of fins look ragged, milky, or bloody. Often follows stress, poor water, or nipping. Improve water quality first. Mild cases may heal with clean, stable water and gentle salt use for tolerant species. Advanced cases may need antibacterial medication.

Columnaris (Bacterial Infection)

White or gray patches that look like cobweb or mold but feel more slimy than fluffy can be columnaris. Lesions often appear around the mouth, back, or fins. This can be fast-moving. Lowering temperature slightly for tropical fish within safe limits and using a suitable antibacterial medication can help, along with very clean water and strong aeration.

True Fungus (Saprolegnia)

Cotton-like tufts on skin, fins, or eggs often follow injury or another infection. Improve water and treat with antifungal medications. Fungal growth commonly indicates a deeper problem that needs attention.

Dropsy (Fluid Retention)

The body swells, scales lift like a pinecone, and eyes may bulge. Appetite drops, and the fish becomes lethargic. Dropsy is not a disease by itself but a symptom of organ failure, infection, or severe stress. Focus on pristine water, gentle salt if safe, and targeted meds where a bacterial cause is suspected, but prognosis is guarded.

Swim Bladder Problems

Fish float, sink, or tilt and struggle to hold position. Causes include constipation, gas, injury, infection, or body shape. Try fasting for a day or two, then feeding a small amount of high-quality, easy-to-digest food. Avoid aggressive water flow. If infection is suspected, consider antibacterial treatment in a hospital tank.

Internal Parasites

Fish lose weight while eating or stop eating and pass long, white, stringy feces. They may become lethargic and dull. A dewormer designed for aquarium fish can help, but treat in a hospital tank when possible. Improve diet variety and cleanliness.

Skin and Gill Flukes

Look for flashing, rapid breathing, clamped fins, and excess mucus. Some fish may develop frayed fins or cloudy patches. A targeted anti-parasite medication is needed. Extra aeration and improved water help the fish cope while treatment works.

Ammonia or Nitrite Poisoning

Fish gasp, hang at the surface, and show red or inflamed gills. Fins may clamp. This often happens in new tanks, after overfeeding, or when a filter fails. Immediate water changes, detoxifiers, and increased aeration are critical. Keep feeding very light until the cycle stabilizes.

Oxygen Deficiency

Multiple fish breathing heavily near the surface or by the filter suggests low oxygen. High temperature, crowded tanks, heavy medications, or clogged filters can cause this. Increase surface agitation, add an airstone, clean filters gently, and reduce temperature within the species’ safe range if it was too high.

Temperature Shock

Fish may go pale, clamp fins, and breathe fast after a big temperature change. Always match water temperatures during water changes and acclimate new fish slowly. Stabilize temps and dim lights to reduce stress.

High Nitrates or Old Tank Syndrome

Chronic stress, lethargy, fin erosion, and poor appetite can come from long-term elevated nitrates and unstable pH. Fix this slowly with small, frequent water changes. Avoid large, sudden swings that can shock fish and bacteria.

How to Tell If It’s the Water, Not a Disease

Signs Pointing to Water Quality

When several fish show stress at the same time and there are no clear spots, growths, or wounds, suspect the water. If a quick test shows any ammonia or nitrite, or very high nitrate, focus on water first. Unusual smells, cloudy water, or a recent overfeeding also support this cause.

Quick Water Rescue Protocol

Do a partial water change of 25–50% with dechlorinated, temperature-matched water. Add extra aeration. Clean the filter sponge gently in tank water, not tap water. Stop feeding for a day. Retest in a few hours. Continue small daily water changes until readings are safe.

Preventing New Tank Syndrome

Cycle the tank before adding many fish. Add fish slowly over weeks, not all at once. Do not rinse bio media in tap water. Keep feeding moderate. Test weekly, and act on any ammonia or nitrite immediately.

How to Examine Your Fish Up Close Without Harm

Safe Netting and Viewing

Use a soft net and guide the fish into a clear container with tank water rather than lifting it in the air. View under good light for a minute, then return the fish calmly. Reduce handling time to lower stress.

Salt Dips and When to Use Them

A short salt dip can help with some external parasites. Only do this if you know your species is salt-tolerant. Use clean, dechlorinated water with the right salt amount and time it carefully. If the fish shows severe distress, end the dip immediately. For beginners, a gentle salt level in a hospital tank is usually safer than dips.

Mucus and Scale Check

Look for extra slime, missing scales, tiny white grains, or gold dust. Note fin edges and gill color. Photos under the same lighting help you track changes over time.

When to Seek a Fish Vet

If you see persistent symptoms that do not respond to careful water fixes and basic treatments, or if many fish die quickly, seek a fish veterinarian. Advanced diagnostics like skin scrapes or gill exams should be done by trained professionals.

Setting Up a Simple Hospital Tank

Equipment and Size

A small bare-bottom tank, tub, or bucket with a lid can serve as a hospital. Use a seasoned sponge filter or add extra aeration. A heater is needed for tropical fish. Provide a cave or plant for cover to reduce stress.

Water and Filtration

Use dechlorinated water. If you have a cycled sponge filter, move it from the main tank to the hospital tank. If not, rely on daily small water changes while monitoring ammonia and nitrite. Keep the environment simple and clean.

Salt, Heat, and Meds

Only add one change at a time unless an emergency demands more. Many medications reduce oxygen, so add aeration. Raise temperature only for issues like ich and only for species that tolerate it. Remove chemical filter media like carbon when using meds that instruct you to do so.

Observation Period

Quarantine new fish for at least 2–4 weeks. For sick fish, continue observation for a full week after symptoms resolve. Record doses and water tests in a notebook.

Medication Basics for Beginners

Read the Label and Active Ingredients

Different brands use different names for the same drug type. Look for the active ingredient and match it to the problem. Follow dosage and duration exactly. Overdosing can kill fish and filter bacteria.

Do Not Mix Medications Blindly

Combining drugs can reduce oxygen and stress fish. Use one targeted med at a time unless the label or a trusted plan says otherwise. Perform water changes and run fresh carbon between different treatments if the label requires it.

Antibiotics, Antiparasitics, and Antifungals

Antibiotics fight bacteria such as fin rot or columnaris. Antiparasitics target ich, velvet, flukes, and internal worms. Antifungals help with true fungus. Some products cover more than one category, but choosing the right class improves success.

The Role of Aquarium Salt

Non-iodized aquarium salt can ease stress, reduce nitrite toxicity, and help with some external parasites. However, many plants, snails, and some catfish or loaches are salt-sensitive. Always research your species before using salt, and measure carefully.

Meds to Keep on Hand

It is useful to keep a reliable ich treatment, a broad antiparasitic, a gentle antibacterial, aquarium salt, water conditioner, and a good test kit. Proper storage extends shelf life. Replace expired products.

Special Notes by Fish Type

Goldfish and Koi

These fish produce a lot of waste and need strong filtration. They are prone to skin parasites, gill issues, and buoyancy problems. Keep temperatures stable and avoid small bowls. Watch for red streaks in fins and treat water problems quickly.

Bettas

Bettas often suffer from fin rot, velvet, and temperature stress. Keep the water warm and steady, and avoid strong currents. Provide gentle filtration, a lid to prevent jumping, and hiding spots. Bettas often benefit from clean, warm water and careful feeding when ill.

Livebearers (Guppies, Mollies, Platies)

These fish can be sensitive to fluctuating hardness and pH. Internal parasites, ich, and fin nipping are common issues. Provide mineral-rich water for species like mollies and keep stocking modest to reduce stress.

Shrimp and Snails

Invertebrates are highly sensitive to copper and many medications. Never treat a display tank that has shrimp or snails with meds that contain copper or other harmful ingredients. Move fish to a hospital tank to medicate, or use invert-safe products.

Catfish and Loaches

Many scaleless fish do not tolerate salt or strong medications. Start at half doses if the label allows and watch closely. Provide lots of oxygen and hiding spots to reduce stress.

Prevention Is Easier Than Cure

Quarantine New Fish

New fish often carry parasites or bacteria that your tank does not have. Quarantine all new arrivals for 2–4 weeks in a simple tank. Observe daily, feed lightly, and be ready to treat before introducing them to your display tank.

Feeding and Nutrition

Feed a varied, high-quality diet in small amounts. Overfeeding leads to poor water and digestion issues. Rotate foods such as pellets, flakes, frozen, and blanched vegetables for species that need them. Remove leftovers after a few minutes.

Maintenance Routine

Do regular partial water changes of 20–40% weekly or as needed. Vacuum debris, rinse the filter sponge in a bucket of tank water, and test parameters. Small, consistent maintenance beats large, rare cleanings.

Stocking and Compatibility

Do not overcrowd. Research adult size and behavior. Aggressive or incompatible fish cause injuries and stress that lead to disease. Provide enough territory and hiding places for shy species.

Stress Reduction

Keep lighting consistent, add decor and plants for cover, and avoid tapping the glass or sudden movements. Use a timer for lights, and match temperature carefully during water changes. Stable conditions mean healthier fish.

When It’s an Emergency

Rapid Breathing and Lying on the Side

If a fish is breathing hard and losing balance, act now. Increase aeration, test water, and perform a partial water change with matched temperature. Keep the room quiet and lights low. If ammonia or nitrite are present, treat the water and repeat small changes.

Massive Ich Outbreak

When many fish show spots at once, treat the whole tank. Raise temperature within safe limits, increase aeration, and start an effective ich medication. Continue treatment for the full recommended time after the last visible spot is gone.

Severe Ammonia Spike

Do immediate water changes, dose a detoxifier that binds ammonia, stop feeding, and add extra aeration. Check your filter, reduce stocking if needed, and monitor twice daily until the cycle stabilizes.

Shipping or New Arrival Stress

Dim lights, float the bag to match temperature, and acclimate gently. Do not feed right away. Add extra aeration and keep the environment quiet. Observe closely for the first 48 hours and be ready to quarantine if symptoms appear.

Putting It All Together: A Simple Action Plan

Observe, Test, Decide

Whenever you think a fish is sick, pause and observe from a distance. Test the water. Decide whether the main cause is environmental or infectious. Fix water problems first. If signs point to a clear disease, move to targeted treatment.

Care First, Meds Second

Clean, stable water and oxygen keep fish alive while you diagnose. Add gentle support like salt only for species that tolerate it. Use medication only when you are confident about the likely cause and always follow the label exactly.

Keep Records and Learn

Write down symptoms, water tests, and treatments. Photos help you spot patterns. Over time you will notice early warning signs faster and treat more effectively with less stress on the fish and the tank.

Conclusion

Telling if your fish is sick starts with knowing what healthy looks like. Watch for early changes in behavior, breathing, appetite, and skin. Always test the water first, because many “diseases” are actually water problems. Use a calm, step-by-step approach: observe, test, isolate, support, and then treat. Keep a simple hospital tank ready, learn the common signs of ich, velvet, fin rot, and other issues, and match treatments carefully to the symptoms. With consistent care and prevention, your fish will recover faster and stay healthier for the long term.