Overnight deaths after the power went out (or flow stopped)
This looks like: dissolved oxygen crashed while circulation stopped
Why this happens
Warm water holds less oxygen. Without surface movement, CO₂ builds and fish suffocate — especially in heavily stocked planted tanks overnight or in summer heat.
How sure are we? High when flow stopped and gasping happened before death — other causes still deserve a quick ammonia check.
What to do right now
- Restore flow immediately — restart filter, wave surface with a clean jug if needed to break film.
- Add temporary surface agitation (battery air pump, second small pump, or aim returns upward).
- Lower temperature a degree or two only if heat is extreme — sudden big chills also shock.
- If many fish: prime fresh dechlorinated water in a bucket and gentle splash surface while power returns.
- Plan: inverter or USB air pump for the next outage; trim stocking if the tank runs “edge of breathless.”
What not to do
- Don’t seal the lid airtight during outages.
- Don’t panic-dose random “oxygen tablets” instead of moving water.
If that doesn’t fix it
Test ammonia after any mass die-off — decaying fish spike ammonia next.
Catch this earlier next time — log your water parameters in App-aquatic. Free forever.