The importance of maintaining good oxygen levels cannot be overstated. Oxygen is vital; not only for the fish but also for the natural filtration processes occurring throughout the pond.
Signs indicating low oxygen levels can include inactive or lethargic fish, fish slow to feed and in extreme cases fish mouthing at the surface around the water return.
Low levels of oxygen can often occur during periods of warm weather, due to the fact that the waters physical capacity for retaining oxygen is reduced. Ponds with large amounts of plants are also at high risk.
Do not assume that ponds will be fine because you have a waterfall or fountain. Often waterfalls and fountains are relatively ineffective because they do not create much of a disturbance at the surface. A popular misconception is that plants are effective, however whilst they release oxygen during daylight hours the process stops after dark.
In an emergency, running some fresh water from the hose into the pond temporarily increase the oxygen level – the best solution is to install a dedicated air pump.