In most cases ponds are at their best during the summer and need relatively little maintenance. Some problems can occur, with the following being the most likely:
Water loss. This is usually caused by evaporation due to the warm temperature. See the question relating to Summer water losses for more details of what to do.
Fish gasping. In hot still conditions oxygen levels in the water can fall to dangerous levels, particularly in heavily planted ponds. Plants produce oxygen in bright conditions through a process called photosynthesis, but use it up in the dark through respiration. If you notice your fish are gasping at the water surface or congregating around any moving water in the morning it may well be that low oxygen levels are to blame. Ensure any waterfalls or fountains are used throughout the night in warm conditions. Filter outflows can also be directed to cause maximum turbulence to help with oxygenation. In severe conditions consider using an air pump, changing part of the water and ensuring that the water surface is not covered in plants.
Plant growth. During the warmer months the growth of fast growing pond plant species can start to choke the pond. Carefully removing some of the faster growing plants to keep part of the pond plant free is advisable. Always compost the plants you remove and never dispose of them into natural watercourses.
Algae growth. Both green water and blanketweed can be a problem in the summer, though usually only if there are too few plants in the pond. Refer to the questions on Why has my pond turned green?and How can I control the blanketweed that grows in my pond? for details of how to control it. In hot weather take care when treating algae. If you use any chemicals that will kill the algae make sure that you remove as much algae as possible before treating, as any that dies will quickly decompose and both pollute and deoxygenate the water.