We would recommend a minimum depth of 45cm (18″) for a wildlife pond or a pond with small fish like goldfish, or for large fish like Koi we would recommend a minimum depth of 90cm (36″) and here’s the reason why.
When you’re building a pond you need to think of how deep it’s going to be, if it’s to shallow the water will get too hot in the summer and could freeze in the winter.
During the summer the water in your pond will heat up a lot faster in a shallow pond than a deeper pond, so your pond needs to be deep enough for your fish to find some cool water lower down to swim in, it’s also good to have a deeper pond to help your fish to hide from predators.
You also need to consider evaporation, if you have a shallow pond the amount of evaporation could affect the water levels more than it would a deeper pond, so if you built a shallow pond to a depth of 30cm (12″) and you lost 5cm (2″) due to evaporation your pond would drop to 25cm (10″), nowhere near the depth a fish would be happy with, on the other hand a 45cm (18″) pond would still be 40cm (16″) which would be fine for your fish.
During the winter months when the water gets cold your fish can hibernate at the bottom of a deep pond where the water is warmer, if your pond is shallow there’s nowhere for the fish to keep warm and get them through the winter.
There’s also the chance your pond could freeze over, a shallow pond could actually freeze solid, but a deeper pond is less likely to freeze at the bottom.
Depth of pond.
Although we recommend a minimum depth a pond should be that doesn’t mean you should stick to it, a depth of 60cm (24″) would be better for smaller fish and a depth of 120cm (48″) for Koi.
The bigger the volume of water you can get in your pond the easier it will be to maintain, plus you can keep more fish, a deeper pond will help you get that extra volume of water if you haven’t got the room to build a large pond.
What fish need is exercise and room to swim, a deeper pond would give more area for your fish to swim, some koi experts recommend a pond depth of 180cm (72″) but if you have a large enough pond there is no need to go this deep.
Also bear in mind when you’re designing your pond it will never be filled right to the top, so dig down a little bit deeper to take this into account.