Tap water went straight in — no conditioner
This looks like: chlorine / chloramine poisoning from untreated tap water
Why this happens
Municipal water disinfectants attack gill tissue. Fish look fine until suddenly they are not — especially right after a big change.
What to do right now
- Dose dechlorinator for the entire aquarium volume at label strength right now — not just the bucket.
- If fish gasp hard, increase gentle surface movement and dim the room.
- Tomorrow, retest ammonia (sometimes chloramine reflects oddly in cheap tests).
- Future: pre-mix change water in a bin with conditioner 15 minutes before pouring.
- Keep a backup bottle — running out is how this repeats.
What not to do
- Don’t assume well water is always chlorine-free — test if you move house.
Stop it happening again
Keep dechlorinator in stock and add it every single time, dosed for the whole tank volume. A spare bottle prevents the 'ran out' disaster that causes this.
If that doesn’t fix it
See sudden-deaths-water-change playbook if temp also diverged.
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.