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.
How sure are we? Very high when untreated tap was confirmed.
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.
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.