Gasping at the surface, filter still running
This looks like: low dissolved oxygen — especially if the tank is warm, crowded, or has a thick surface film
Why this happens
Oxygen enters at the air-water boundary. Oil films, lily pads of duckweed with zero ripple, and summer heat all starve fish at the surface first.
How sure are we? High when gasping clusters at the top, equipment still humming.
What to do right now
- Ripple the surface gently — aim the filter return up, add an air stone if handy.
- Remove thick floating mats blocking exchange for now.
- If heat wave: shade the glass, float fan across surface (not into water), or partial change with matched cool water carefully.
- Check stocking — the calculator helps when headcount grew slowly over months.
- Test ammonia anyway — sometimes gasping is also poison; oxygen fixes do not replace that answer.
What not to do
- Don’t seal the tank airtight “to hold humidity.”
If that doesn’t fix it
Persistent gasping with clean tests could be flukes on gills — see flashing diagnosis.
Catch this earlier next time — log your water parameters in App-aquatic. Free forever.