Sudden deaths after a water change
This looks like: chlorine / chloramine poisoning or temperature shock from the new water
Why this happens
Tap water often holds chlorine and chloramine that burn gills within hours. If dechlorinator was skipped or under-dosed, fish can die fast. Cold replacement water can also shock warm-water fish even when chemistry reads “fine.”
How sure are we? This pattern matches most “everyone was fine until the water change” emergencies we see — still confirm with a liquid test kit when you can.
What to do right now
- If any fish are alive: add the correct dose of fresh dechlorinator for the whole tank volume immediately, then dose again for the next partial change.
- Match replacement water to tank temperature within about 1°C (2°F) using a thermometer — float the change bucket first.
- Do a smaller follow-up change (20–25%) only with temperature-matched, properly dechlorinated water — not another cold slug from the tap.
- Increase surface rippling gently (air stone or filter outlet) to help stressed fish breathe while gills recover.
- Tomorrow, test ammonia, nitrite, and pH; write the numbers down so the next move is based on data.
What not to do
- Don’t “super-clean” the filter or strip the tank in panic — that often makes chemistry swing harder.
- Don’t stack random medications while water quality is unknown.
If that doesn’t fix it
If fish keep dying after changes are matched and dechlorinated every time, test tap water for ammonia (some taps contain chloramine/chlorine that needs full-volume dose) and check a cheap heater thermostat.
Catch this earlier next time — log your water parameters in App-aquatic. Free forever.