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.
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.”
Stop it happening again
Keep the surface rippling, don't overstock, clear any surface film, and add extra aeration in warm weather when water holds less oxygen.
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.
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.