Keeping your pond cool and oxygenated should be your primary focus during the hot summer months. The warm weather has a dramatic affect on oxygen levels in your pond - the warmer the water, the lower the level of dissolved oxygen. Low oxygen levels can lead to fish kills, increased algae growth, and a reduction in beneficial bacteria resulting in an increase in ammonia, nitrites and nitrate.
A relatively easy way to keep your pond cool is to add surface plants to the pond). If you have fish, a good goal is to have 70% of your pond surface area covered during the hottest weeks. You can add floating plants, such as water hyacinth, water lettuce, duck weed and water chestnut. Another option is to add a floating planter, or floating planting baskets (see attached floats available on eBay), which provide much more plant variety.
Also consider using a pond tint. These all-natural products act as a "sun block" for you pond - keeping temperatures lower and inhibiting algae growth. A great choice is the Pond Logic Plus (that includes the PondClear clarifying agent) which comes in either Nature's Blue or Twilight Blue Tint.
A bottom aeration system is extremely effective in cooling the pond water and increasing oxygen levels thereby ensuring healthy pond life, promoting beneficial bacteria growth, and lowering toxic gases and ammonia by promoting gas exchange. Double your aeration in the hot summer months.
Great idea Phil. Do you know if duckweed would be effective from this perspective?
Darn it, I forgot to mention duck weed. Duck weed is great. I use duck weed in my pond and in my large IBC sand filter sumps. I also use water lettuce and hycinth in my large IBC sand filter sumps to culture beneficial bacteria. I have 4 to 5 gold fish in the sumps pooping away and wasting food that deteriorates to aid in the bacteria growth.
I went back and added duck weed. Thanks Jere.