Putting rocks or gravel on the bottom of a koi pond is probably one of the most common mistakes new pond builders make. Admittedly, they do look beautiful when they are first put in. However, there are some big problems that quickly develop.
- Rocks prevent ﬁsh waste from getting to the bottom drain because any ﬁsh waste that falls to the pond’s bottom is trapped in the rocks. Now, instead of being swept away into your pond’s ﬁlter where it can easily be removed, this waste sits there, creating a sludge on the bottom of your pond. This sludge can degrade your water quality and provide a home for harmful bacteria and parasites. The only way to remove this sludge is to remove and store the koi, drain the pond, and vacuum the sludge out of the rocks on the pond’s bottom. As you can imagine, this is not an easy job.
- Additionally, rocks will trap leaves and debris on the bottom. As they decompose, most leaves give off tannin acid. Tannin acid will not bother the koi, but it will turn the pond water a root beer color. You see this often with a pond or a stream that has rocks or gravel on its bottom. You can perform a 100% water change, but within a week or so, the water will return to the root beer color. Just as with ﬁsh waste, the only way to alleviate this problem is to drain the pond and thoroughly wash the rocks.
You can avoid all these problems by keeping rocks off of the bottom of the pond. Creating a shelf around the sides of the pond extending 12 to 18 inches below the water surface and then installing rocks on this shelf can create a beautiful effect. After filling the pond with water, the shelf gives the illusion that the rocks extend to the pond bottom. Now, if you keep the pond bottom free of rocks or gravel, and you contour the bottom so that the drain is at the lowest point of the pond with a gradual slope leading to it from all sides, you will have the best of both worlds. Your pond will look beautiful and be easy to maintain.