Fish Dying After a Water Change – Urgent Checks

What to check first

• Confirm replacement tap water was conditioned.
• Check temperature difference and equipment operation.
• Test ammonia and nitrite if fish are stressed.
• Check ammonia and nitrite immediately.
• Check temperature, filter and pump operation.
• Increase aeration and inspect for a recent environmental change.

What you can do now

• Increase aeration and confirm the filter and pump are working.
• Test ammonia, nitrite and temperature.
• Review all recent water, product, feeding and cleaning changes.
• Prioritize environmental causes when several fish are affected quickly.

What may be contributing

• Untreated replacement water may be associated
• Temperature mismatch may be associated
• A sudden water-chemistry change may be associated
• An environmental or water-quality problem
• Equipment or oxygen interruption
• Temperature change
• Possible contamination
• A fish-health problem that needs specific identification

Questions that help narrow the problem

Was tap-water conditioner used?

Yes · No · Not sure

Were fish affected immediately?

Yes · No

Was replacement-water temperature matched?

Yes · No · Not sure

Do you have ammonia and nitrite results?

Yes · No

How many fish have died?

One fish · Several fish · Most or all fish

Over what period?

Within hours · Within 1–2 days · Over several days

What changed recently?

Water change · New fish · Medication added · Conditioner or product added · Filter cleaned or media replaced · Power failure or pump stopped · Large amount of food · Cleaning chemical may have entered water · Nothing known

What are surviving fish doing?

Gasping · Staying at the surface · Lying at the bottom · No obvious symptoms

Relevant tests and product guidance

These links come from the existing troubleshooting record. They do not replace water-quality correction or the current product label.

• API Tap Water Conditioner
• Seachem Prime
• Nutrafin Aqua Plus