Nothing changed — but fish died anyway
This looks like: hidden equipment failure (heat runaway, filter stopped, or air line blocked)
Why this happens
Cheaper heaters sometimes cook tanks or shut off; canister impellers seize quietly. By morning you only see dead fish and “nothing happened.”
How sure are we? One of several possibilities — temperature first, then flow, then ammonia from a stalled filter biofilter.
What to do right now
- Measure tank temperature now vs your target — hot glass or >3°F drift signals a stuck heater (unplug suspect heater first).
- Check filter output with your hand — weak trickle means clean impeller cavity per manufacturer guide.
- Test ammonia and nitrite immediately after any filter stoppage — biofilter can stall within a day.
- For heat issues: cooled dechlorinated water slowly, separate redundant heater or lower room temp short-term.
- Replace ageing heaters before monsoon seasons; keep a cheap spare air pump if the power blips often.
What not to do
- Don’t crank the heater to “fix” a chiller problem — diagnose first.
If that doesn’t fix it
If ammonia rises after filter failure, treat like a mini-cycle with measured changes and reduced feeding.
Catch this earlier next time — log your water parameters in App-aquatic. Free forever.