Koi are the fish that are most commonly associated with garden ponds. They have been raised in the Orient for centuries, with the earliest colored varieties dating back two hundred years. Due to the size of these fish they should be kept in large sized ponds, especially if you plan on keeping more than a few fish. Koi have strong mouths, and like to dig in the substrate. Therefore, only tough leafed plants should be planted in the pond, and be sure to protect the roots and soil of your pots with a layer of gravel.