|What is the ideal water temp for koi /and/or goldfish living in a pond and what temperature range can they withstand? We have recently installed a 300 gallon pond in our yard; it receives morning & early afternoon sun. The water temp today was 91 - I am afraid I will have boiled fish! How can I cool the water to an acceptable range ( I have 1 lily plant in the pond)?
Ideally, koi and goldfish would be at seventy four degrees Fahrenheit all year around. In a perfect world, this would balance nicely their metabolism, appetite, and the corresponging metabolic function of the beneficial nitrogen reducing bacteria in their filter. Much warmer water creates considerable headaches as far as dissolved oxygen, not to mention that Koi colors blanch in overly warm water. Much cooler than seventy four degrees and then you'll notice that the fish lack energy due to their clower metabolism, but most significantly, the beneficial nitrogen reducing bacteria in the pond (and filter) will not effectively reduce the nitrogen produced by the Koi and then you'll have nitrogen accumulations in the form of Ammonia and Nitrite.
When a pond gets very very warm in summer, it's not good. Dissolved oxygen becomes a problem and then if the pond water temperature swings down sharply at night, (enhanced by waterfalls in a phenomenon called "supercooling" - this effect becomes stressful and the fish become more vulnerable to diseases.
You should SERIOUSLY consider building an attractive shade-cover for overhead; to shade the pond. Two upright 4x4's planted in the groun could support a square frame onto which standard 2x6 lumber is stood on it's edge, oriented in parallel like a grid which will admit some sunlight at midday and filtered light the rest of the day. This is consistent with Japanese shade shelters. If even more effective if you put weatherproof shadecloth across the top of the shelter.
If you're lucky enough to have free, non chlorinated water such as you might get from a well, then you could run this water into the pond over the day at a slow rate to keep water temperatures from climbing over eighty two degrees. I would consider that the highest "safe" water temperature.
By the same token, I've a friend in Vietnam who kept Koi in a wheelbarrow at over 100 DF and all he did was change the water every day in the river.