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.
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.
Stop it happening again
Keep a battery or USB air pump for outages, avoid overstocking a warm tank, and make sure the surface always ripples. Heat waves are the classic trigger, so plan ahead for them.
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.
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.