What is the difference between a water feature, water garden and Koi pond?

The difference between a water feature, a water garden, and a Koi pond is the focal point and providing the right environment and aesthetics for each. A water feature has water itself as a focal point...waterfall, fountain etc. They can or cannot support life but if the water movement is the focus then they can be chlorinated and one never needs to worry about algae adversely affecting the feature.

A water garden has plants as a focal point. They are relatively shallow and require minimal filtration and can support limited aquatic life.

A Koi pond on the other hand has Koi as a focal point. Koi requirements are beyond what a typical water garden can support. Much more filtration is required to process the waste from the Koi. Obstructions within a water garden can also adversely affect Koi, compounding the chance of injury and resulting diseases.