New Fish Dying in the First Days: Causes and Prevention

Why newly bought fish die within days: transport stress, ammonia and pH shifts in the bag, acclimation mistakes, latent disease and mismatched water, plus quarantine steps that cut losses.

New fish that die within the first days have usually been weakened by transport and then exposed to a second stress: degraded bag water, a fast change in temperature or pH during acclimation, water chemistry they are not suited to, or an infection they were already carrying. Separate quarantine and slow, measured acclimation remove most of these risks.

What the symptom looks like

Losses may begin within hours, pointing to a water or handling fault, or after several days, often with visible disease. Fish gulping at the surface (piping) indicate low oxygen, according to the MSD Veterinary Manual. A UF/IFAS fact sheet on ornamental fish shipping lists ragged fins, ulcers, fungal patches, white spots, excess mucus, scale loss and abnormal behaviour as the signs to look for when fish arrive.

Likely causes and how to confirm each

1. Transport stress

UF/IFAS notes that stress during harvest, holding and transport weakens the immune system and increases susceptibility to disease. Fish shipped by road may also spend days without food: a 2025 study of male Betta splendens after two days of overland transport found that growth recovered with refeeding over about eight days. Losses that begin after a few days, with no water fault, often trace back to this weakened state combined with a pathogen.

2. Ammonia and pH shift in the bag

During shipment, oxygen in the bag falls while carbon dioxide and ammonia from respiration and waste accumulate, according to UF/IFAS. Dissolved carbon dioxide lowers pH; the MSD Veterinary Manual describes how vigorous aeration of carbon dioxide-rich water raises pH, and how the toxic un-ionised share of total ammonia increases as pH rises. Once a bag is opened and the gas escapes, the same ammonia becomes more toxic. Testing the bag water for pH and ammonia on arrival shows how severe the shift is.

3. Acclimation errors

The MSD manual warns that sudden temperature changes of even a few degrees can compromise immunity and gives about 1 °C per hour as the maximum recommended change during handling. Dropping fish straight into a tank at a different temperature, or leaving them for a long time in opened bag water, adds stress at the moment they are weakest. The article "Acclimating New Fish" describes the methods step by step.

4. Incompatible water

Temperature requirements are species-specific: some fish tolerate wide ranges while others, such as discus, need a narrow window, and most freshwater fish have lethal pH limits of roughly 4 and 10. Fish moved between very different pH, hardness or salinity can fail even when ammonia and nitrite are zero. Compare the parameters of the tank with the documented needs of the species before purchase.

5. Latent disease

Fish can carry infections without showing signs. The FAO quarantine manual explains that holding new animals in isolation gives time for diseases or infections to become evident, that stress can bring subclinical infections into the open, and that ornamental fish play a significant role in spreading transboundary aquatic animal diseases. Deaths that start several days after arrival, with clean water test results, point to this cause.

Immediate safe actions

• Test the tank for ammonia, nitrite, pH and temperature to rule out a water fault.
• Remove dead fish quickly and keep the survivors under observation, ideally in a separate tank.
• Note all signs and the timing of each death; if losses continue, contact an aquatic veterinarian or fish health laboratory before treating, as treatment can interfere with diagnosis.

Acclimation steps that reduce losses

1. Equalise temperature first, letting it change gradually rather than in one step.
2. After opening the bag, keep the time fish spend in the old bag water short, because ammonia becomes more toxic as carbon dioxide escapes.
3. When shop and home water differ markedly in pH or hardness, avoid an abrupt switch; Practical Fishkeeping notes that sudden chemistry changes stress fish.
4. Transfer the fish without the transport water, which can carry pathogens and waste.

Quarantine steps

• Hold new fish in a separate tank with its own net, bucket and siphon; the FAO manual advises against shared recirculating water because it spreads pathogens between tanks.
• Cover tanks so splashes from aeration cannot carry pathogens to neighbouring tanks.
• Disinfect equipment before it is used on another system.
• Observe daily for signs of disease and record deaths.
• Quarantine length is a risk decision: the FAO manual cites official minimums of six weeks for freshwater and three weeks for marine ornamental shipments in New Zealand, and 7 to 21 days for freshwater ornamental finfish in Australia, noting that risk falls as the period lengthens.

The article "Quarantine Tank for New Fish" covers the set-up of a quarantine system.

When it is disease

If water tests are normal and fish show spots, ulcers, fin erosion or wasting, the cause is most likely infectious. Use the disease article matching the signs; for repeated losses, a fish health laboratory can examine live, sick fish to identify the pathogen.