As a Certified Aquascape Contractor, we communicate daily with other contractors across the country to share information, tips, and tricks. Last season, a fellow CAC posted pictures of a warm water fish die off that was devastating to the client.
Fish need oxygen to live and concentrations of dissolved oxygen are much lower in warm water. In the dog days of summer, cold water fish can have a hard time getting enough oxygen. In natural bodies of water, the temperature can have devastating results. Salmon, for example, become stressed at 68 degrees and stop migrating at 74. Koi deaths will begin when dissolved oxygen levels fall below 3ppm.
It is important to address potential temperature issues when initially designing a pond. If the pond is in direct sunlight, consider a shade tree. Water lily pads will assist in shading the water and providing some habitat. The recommendation of 40-60% of the pond being covered in aquatic plants isn't just to balance nutrients, it is to buffer the temperature as well. If you are in a very hot climate, consider extra fish caves and dig a little deeper. Your beneficial bacteria need oxygen too. If you are in full sun, consider adding aeration during the hot summer months so your nitrogen levels don't get out of control.