If your fish are dying and you cannot see anything wrong with them, start with the first entry below: sudden death with no visible symptoms is almost always a water problem, not a disease. If you can see a specific sign, jump to the matching symptom. Each entry lists the likely causes in rough order, the first thing to do, and the treatment pathway. Test your water before treating anything. Ammonia, nitrite, and low oxygen mimic disease and get misdiagnosed constantly.
Why are my fish dying with no symptoms, or all at once?
Fish are dying but you cannot see anything wrong with them. No spots, no fungus, no fin damage. Often several die over a day or two, frequently in a tank set up within the last couple of months. Healthy-looking fish found dead, or sitting on the bottom breathing hard before they go.
Likely causes
1.Ammonia or nitrite spike (new tank syndrome)
The number one killer of fish with no visible disease. In a tank that is not fully cycled, ammonia and nitrite climb to toxic levels and poison fish from the inside. New tanks, and tanks that lost their cycle from the filter being off too long or over-cleaning, are the usual victims.
Treatment guide: How to Lower Ammonia in Your Aquarium → 3.Chlorine or chloramine (no dechlorinator)
Tap water used straight, or a water change without conditioner, burns gills and can kill within hours. Chloramine does not gas off the way chlorine does and needs a conditioner rated to handle it.
4.Temperature swing or low oxygen
A heater that failed or stuck on, a cold water change, or a hot room can swing temperature enough to kill. Warm water also holds less oxygen, so a crowded or under-aerated tank can suffocate fish overnight.
5.Old tank syndrome (chronic high nitrate)
In an established tank gone too long without water changes, nitrate creeps up slowly and the fish adapt. Then one large water change shocks them, or one more fish tips it over. Long neglect, then sudden deaths.
Treatment guide: How to Lower Nitrates in Your Aquarium →
First thing to do
Do a 50% water change right now with dechlorinated, temperature-matched water. It dilutes whatever is poisoning them while you find the cause. Then test ammonia, nitrite, and nitrate before doing anything else. Do not reach for medication. This is almost never disease.
Diagnostic steps
- Test ammonia and nitrite first. Any reading above zero on either is very likely your answer.
- Test nitrate. Above 40 ppm in an older tank points to old tank syndrome and overdue water changes.
- Confirm you used a dechlorinator on your last water change. If not, chlorine or chloramine is a prime suspect.
- Check the heater and a separate thermometer. A stuck or failed heater swings temperature enough to kill quietly.
- Ask how old the tank is. Deaths in a tank under two months old almost always mean it was never cycled.
- Only once the water tests clean and stable should you look at disease. Use the symptom entries below.
My fish has white spots - what is it?
Tiny white grains scattered on the body, fins, and gills. Usually the size of a grain of salt. Fish often flashes against objects before the spots appear.
Likely causes
3.Fungus
Cotton-tuft texture, not a flat dot. Usually on a wound or fin edge.
First thing to do
Raise temperature to 82°F (slowly, 1°F per hour). This speeds the parasite's life cycle and exposes it to treatment. Do not add medication before confirming which one you're seeing.
Diagnostic steps
- Look at the fish under a flashlight. Ich dots are discrete and white. Velvet looks like powdered gold dusting.
- Check breathing rate. Velvet attacks gills first and causes fast gill movement before visible signs.
- Test ammonia and nitrite. A stressed, poorly-cycled tank is what let the outbreak take hold.
- Pick the treatment guide that matches the diagnosis and follow the full duration - stopping early is why ich comes back.
My fish is gasping at the surface - what's wrong?
Fish hangs near the waterline, mouth working at the air-water boundary, or cruises the surface with gill covers flaring.
Likely causes
1.Low dissolved oxygen
Warm water holds less oxygen. A crowded tank, a dying filter, or high temperature can strip it out fast.
First thing to do
Do a 50% water change with dechlorinated, temperature-matched water. Add an airstone or point the filter outflow at the surface to break it. Then test ammonia and nitrite.
Diagnostic steps
- Test ammonia, nitrite, nitrate. Any reading above zero on ammonia or nitrite is the likely cause.
- Check water temperature. Above 82°F oxygen drops sharply.
- Watch whether all fish are gasping (water issue) or just one (disease, gill parasite, individual problem).
- Increase surface agitation. This is the fastest oxygen fix you can make.
Why are my fish's fins clamped?
Fins held tight against the body instead of spread. Fish often hangs in a corner and ignores food.
Likely causes
1.Water quality stress
Ammonia, nitrite, wrong temperature, or wrong pH all produce clamped fins as a first sign before visible disease.
3.Bullying or stress from tankmates
A subdominant fish in a too-small tank will clamp up permanently.
First thing to do
Test water. Do a 30% change if anything is off. Watch for secondary signs over 24 hours - spots, flashing, fin damage - because clamping alone is not enough to pick a treatment.
Diagnostic steps
- Test ammonia, nitrite, nitrate, pH, temperature.
- Observe for 24 hours. Clamping with no other signs almost always means water or stress.
- Check tankmate aggression patterns, especially at feeding time.
- If spots, flashing, or color loss appear, treat for the matching parasite.
My fish has red streaks in its fins or body - what causes that?
Thread-thin red lines running through fin rays or across the body. Fins may also look bloody at the edges.
Likely causes
2.Bacterial septicemia
A systemic bacterial infection. Red streaking plus lethargy and loss of appetite points here.
3.Physical injury
One clean red streak with no other signs is often just an injury healing. Watch for secondary infection.
First thing to do
Test ammonia and nitrite first. A 50% water change buys time. If water is clean, the fish likely needs an antibiotic like kanamycin or a combination treatment - but do not medicate without confirming water is clean, because antibiotics in dirty water crash the cycle.
Diagnostic steps
- Test ammonia and nitrite. Any reading is almost certainly the cause.
- If water is clean, observe appetite and activity. Septicemia kills appetite within a day or two.
- Isolate if possible to a quarantine tank before medicating.
- Treat with a broad-spectrum antibacterial only after ruling out water quality.
Why are my fish's gills moving so fast?
Gill covers pumping visibly faster than the other fish in the tank. Often one-sided movement if a fluke is on one gill.
Likely causes
1.Gill flukes
Fish may also scratch against objects. Flukes are microscopic but the effect is obvious.
4.Low oxygen
Entire tank gasps. Check temperature and surface movement.
First thing to do
Test ammonia and nitrite. Increase surface agitation. If water is clean and only one fish is affected, suspect flukes or velvet and move on to treatment.
Diagnostic steps
- Test water parameters first.
- Count respirations per minute and compare with healthy tankmates.
- Look for secondary signs - spots, dusting, clamped fins.
- Treat flukes with praziquantel if flukes are confirmed.
My fish keeps scratching against rocks and decor - why?
Fish darts and rubs its sides against substrate, rocks, or plants. Sometimes called flashing. Repeats every few minutes.
Likely causes
2.pH shock or swing
A sudden pH change irritates slime coat. Common after large water changes with untreated tap.
First thing to do
Test water. If parameters are fine, watch for white spots within 48 hours - flashing often precedes visible ich by a day or two. Raise temperature to 82°F to push the parasite cycle forward.
Diagnostic steps
- Test ammonia, nitrite, and pH.
- Observe for 48 hours. Spots usually appear after persistent flashing.
- If spots appear, treat for ich or velvet based on appearance.
- If no spots ever appear and parameters stay clean, consider flukes and treat with praziquantel.
My fish's scales are sticking out - what is pineconing?
Scales lift away from the body, giving the fish a pinecone silhouette when viewed from above. Belly often swollen.
Likely causes
1.Dropsy (internal bacterial infection with organ failure)
Pineconing is the outward sign of fluid buildup inside the body. Late-stage and usually fatal.
2.Kidney failure from chronic poor water
Same presentation, different trigger. The fish has been sick longer than you noticed.
3.Viral or parasitic cause
Rare but possible. No practical home treatment.
First thing to do
Move the fish to a hospital tank if you can. Survival odds are low by the time pineconing is visible. Kanamycin with epsom salt (1 tbsp per 5 gallons) is the standard attempt. Be honest with yourself about whether continuing treatment is kind.
Diagnostic steps
- Isolate the fish to protect tankmates.
- Epsom salt bath to reduce fluid pressure.
- Broad-spectrum antibacterial (kanamycin or a combination).
- Reassess daily. Euthanasia is a legitimate call if the fish stops eating and sinks to the bottom.
My fish is bloated - what's wrong?
Rounded, swollen belly. Fish may also list to one side or struggle to stay level.
Likely causes
1.Constipation / overfeeding
The most common cause by a long stretch. Dry pellets expand in the gut.
2.Swim bladder disorder
Fish cannot maintain depth. Often caused by constipation but can be structural.
3.Early dropsy
Bloating with no scale lifting yet. Treat fast because the window closes quickly.
4.Pregnancy (livebearers only)
Female guppies, mollies, platies, swordtails carry visibly before giving birth.
First thing to do
Fast the fish for 3 days. On day 4, feed a small piece of blanched, deshelled pea. Do not medicate for dropsy unless scales start lifting.
Diagnostic steps
- Stop feeding for 3 days.
- Watch swimming behavior. Listing or floating upside down points to swim bladder.
- On day 4, try a blanched pea or daphnia.
- If bloating progresses or scales lift, treat as dropsy.
My fish's fins are fraying - is this fin rot?
Fin edges look torn, ragged, or white at the tips. Fin tissue recedes toward the body over days.
Likely causes
2.Fungal infection on damaged fins
Cotton-like tufts on fin edges rather than recession. Often follows injury.
3.Physical damage from tankmates or decor
Clean tears, no inflammation, no discoloration. Leave it and let the fish heal.
First thing to do
Test water and do a 30-50% water change. Remove the cause (decor with sharp edges, nippy tankmates). Clean water alone heals mild fin rot in a week or two.
Diagnostic steps
- Test ammonia, nitrite, nitrate.
- Examine fin edges under light. Ragged and receding = bacterial. Cotton tufts = fungal.
- Improve water quality before reaching for medication.
- If recession continues after a week of clean water, treat with an antibacterial.
My fish has a bulging eye - what is popeye?
One or both eyes protrude from the socket. Can look cloudy around the bulging eye.
Likely causes
1.Bacterial infection behind the eye
Bilateral popeye (both eyes) usually means a systemic bacterial infection.
2.Physical injury
One eye only, no other signs, often resolves on its own with clean water.
3.Poor water quality
Chronic nitrate or ammonia exposure causes popeye as a late sign.
First thing to do
Test water, do a 30% change, and dose epsom salt at 1 tbsp per 5 gallons to pull fluid out of the eye. If both eyes are affected or other signs appear, treat with a broad-spectrum antibacterial.
Diagnostic steps
- Test water.
- Check whether one or both eyes are affected.
- Inspect decor for sharp points the fish may have hit.
- Treat with antibacterial only if bilateral or paired with lethargy.
There's white cotton growing on my fish - is it fungus or columnaris?
White, cotton-like or fuzzy patches on the body, mouth, or fins. Can spread in hours (columnaris) or over days (fungus).
Likely causes
1.True fungus (Saprolegnia)
Cottony, three-dimensional tufts. Usually on wounds or dead tissue. Slower spread.
2.Columnaris (bacterial, not fungal)
Looks fungal but is bacterial. Starts as a saddle-shaped white patch or mouth fuzz. Can kill in 24-72 hours.
3.Epistylis (protozoan)
Short white tufts that do not respond to antifungals. Needs antibacterials.
First thing to do
If growth spread visibly in under 24 hours, treat as columnaris with kanamycin or a nitrofuran-based medication. If slow and tuft-like on an injury, treat as fungus with methylene blue or a standard antifungal. Wrong diagnosis costs the fish.
Diagnostic steps
- Time the spread. Fast = columnaris, slow = fungus.
- Look at texture. True fungus is three-dimensional; columnaris is flatter and saddle-shaped.
- Check mouth first. Mouth fuzz is nearly always columnaris.
- Pick the matching treatment and dose for the full course even if symptoms fade early.
My fish is floating sideways or can't swim upright - why?
Fish tilts to one side, floats belly-up, sinks to the bottom unable to rise, or cannot maintain depth.
Likely causes
1.Swim bladder disorder from constipation or overfeeding
Fancy goldfish and bettas are the usual victims. Diet fix handles most cases.
2.Severe bacterial infection
Usually paired with other signs - lethargy, color loss, clamped fins.
3.Neurological or genetic issue
Especially in heavily-bred fancy strains. Sometimes permanent, sometimes manageable with a low water line.
First thing to do
Fast for 3 days and feed a blanched pea on day 4 if the fish can still eat. Lower the water level so the fish doesn't exhaust itself fighting to surface. Test water to rule out poisoning.
Diagnostic steps
- Stop feeding for 3 days.
- Lower water level to reduce effort.
- Test ammonia and nitrite.
- If no improvement in a week and water is clean, swim bladder may be permanent - decide between supportive care and euthanasia.