Fish can last weeks without food, but oxygen is essential. Summer is probably the only time you'll need to worry about oxygen levels in the pond if you have an ecological pond with good circulation and filtration.
In the warmer months, fish activity increases dramatically, increasing their oxygen requirements. Stress from living in water that is too warm (and hence low in oxygen) can make your fish vulnerable to illness.
Advice on maintaining a cool pond
Provide shaded area's with shade cloth or setup a gazebo at one end of the pond.
It's important to remember that any food that goes uneaten throughout the summer will quickly decompose in the pond's warm water reducing your water quality. Feed a high quality fish food such as hikari.
Clear the bottom of the pond of any debris (leaves, sticks, etc.) to keep your water quality high.
Cut back any plants that are covering the water's surface to allow more oxygen to enter the water.
Adding an extra pump or doing more water changes on really hot days will assist in keeping the water quality high and cooler.
Because of photosynthesis in plants,the lowest concentration of oxygen occurs i the early morning.Plants release oxygen during the day and fish use it at night. Water temperatures in the summer can exacerbate this problem. Because of this, you need to always have your pond's pump running to keep the water moving.
Having constant water flow and movement introduces more oxygen into the water. You could also add a fountain or a waterfall.
As an added bonus to maintaining a healthy aquatic environment for your fish this summer, your preparing your pond & fish for the coming transition to Autumn and winter.
Windsor fish hatchery Breeding and Selling Koi & Goldfish since 1984