Summer problems in guppy tanks often look like sudden illness: fish gather at the surface, breathe fast, lose colour, and sometimes die quickly. The hidden cause is often not a parasite — it’s oxygen. Warm water holds less oxygen. At the same time, fish metabolism increases and they demand more oxygen. Add heavy feeding, crowded tanks, and warm nights, and you can get an oxygen crash even if your test kit looks “fine.”
Signs of oxygen trouble
- gasping at the surface or hanging near filter output
- rapid gill movement
- lethargy and loss of appetite
- fish improve quickly when you add aeration (a big clue)
Why summer makes it worse
- Less oxygen solubility: warm water physically holds less.
- Higher demand: fish and bacteria use oxygen faster.
- Night-time risk: oxygen can drop overnight when rooms stay warm.
Prevention plan
- Increase surface agitation: point the outlet at the surface.
- Add an airstone: cheap, effective, and immediate.
- Feed lighter in heat: less waste reduces oxygen demand.
- Keep lids ventilated: allow gas exchange (while staying secure).
Emergency response
If fish are gasping, add aeration immediately, reduce temperature if safe to do so gradually, and do a moderate water change with temperature-matched conditioned water. Oxygen first. Many fish recover quickly when oxygen is restored.
Summer oxygen issues are preventable. If you plan for oxygen, your guppies stay calm, colourful, and far less likely to crash during heat spikes.
