Fish gasping at the surface is usually the first sign something’s wrong with dissolved oxygen, the oxygen present in pond water that fish breathe through their gills.
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Why It Drops
Oxygen enters water mainly through surface agitation and plant photosynthesis. Warm water holds less of it than cool water, and at night plants stop producing oxygen and start consuming it, which is why oxygen crashes tend to happen after dark in hot weather.
The Fast Fix
Running a pump, fountain, or air stone to agitate the surface is the quickest way to raise oxygen levels in an emergency, well before any chemical treatment would help.
