I currently have two young koi fish, and they need a bigger pond. My dad and I are planning on making one in our backyard, but it gets incredibly hot in the summer. Is this dangerous to the koi, and is there a way to avoid it?
It will mainly depend on pond exposure to the sun, it's location and construction, meaning that if you don't have enough water, enough shade and a good ratio fishes-water volume you will need to be extra careful when the sun hits.
A good spot for your pond is where it takes sun only during morning. It will give enough light to your plants (4-6hs) and keep temp cool.
Although if there is no way you can skip direct sun you will need to be aware of water temp but most important your koys behavior to react and cool the temp by changing some amount of water (10 % fresh water and don't forget to dechlorinate).
Good luck with your new pond!