Oxygen is as essential to fish as human, air-breathing beings.

The level of oxygen that it is possible to maintain in water drops radically as the temperature of that water rises. At night in a pond, when the oxygenating plants fail to release oxygen as a by-product of photosynthesis, but instead produce carbon dioxide, it becomes critical that the ecosystem that is established in your pond does not have any further pressures upon it. If there is, for instance, a lot of bacterial activity in the bottom of the pond in the breakdown of waste and organic matter, it will further deplete the limited resource of oxygen.

Lack of oxygen: EFFECTS -. Gasping at the surface especially on hot, humid overcast evenings.

The oxygen content will be at its lowest between 6am and 7am - test then (It needs to be 1mg/litre).

EMERGENCY TREATMENT. Oxygenate with an air pump, venturi, fountain, waterfall or, if necessary, a hose with force from a height.

Possibly too many plants, if so reduce. If the only plants present are algae, then partial water change with conditioner. Then treat as for nitrates with a view to cleaning out.

Too much oxygen! EFFECTS - Fish get bubbles in their blood and fins. It has the same effect as 'the bends' in divers. They become bloated with their scales sticking out resulting in a similar appearance to the symptoms of dropsy.

CURE Gently run in fresh water from a hosepipe.