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.”
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.
Stop it happening again
Pre-mix and dechlorinate your change water in a bucket about 15 minutes before it goes in, and match its temperature to the tank. Never pour straight tap water into a stocked tank.
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.
This isn't veterinary advice - it's a directional read to help you decide your next step, based on the most common causes. We can't see your fish, so pair it with a liquid test kit and clear photos, and see an aquatic vet or a trusted local fish shop for a valuable or seriously ill fish.