Pond Rocks & Gravel

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There are two types of rock that are commonly used to build an ecosystem pond:

1) Boulders and Larger Rocks.

We like to build up the walls of our ponds with larger rocks. (See the diagram above.) It not only makes a pond look much more natural, but it also helps give some structure to the walls to prevent the dirt from falling in over time. If the wall is 24” or less, you don’t have to use concrete mortar to set the rocks. Instead of concrete mortar, shoot some expanding waterfall foam in and around each rock, which will help lock them into place and keep them from moving around. If you’re building a deeper pond that has a section of wall taller than 24”, it’s usually a good idea to use some concrete mortar around the rocks as you build up the wall. You can even stain the concrete mortar above the water surface later to match the color of the rocks if you like. When we talk about a wall that’s 24”, we mean the height of a vertical section, not the total depth of the pond. In the above pond diagram, the top shelf wall would be 8”, and the lower wall would be 16”. So in this example, the concrete mortar wouldn’t be needed.

2) Gravel and Smaller Rocks.

One of the main differences between a dedicated koi pond and an ecosystem pond is that an ecosystem pond has a thin layer of gravel on the bottom to assist in colonizing beneficial bacteria for natural biological filtration. In contrast, dedicated koi ponds usually don’t have any gravel at all. The main reason dedicated koi ponds don’t have gravel is they often use bottom drains to remove all solids from the bottom of the pond. Having gravel on the bottom would only get in the way of the function of the bottom drain. Instead of using gravel, dedicated koi ponds make up for the loss of beneficial bacteria with larger filtration systems.

Since we’re building an ecosystem pond and not a dedicated koi pond, we highly recommend putting gravel and/or small rocks on all horizontal surfaces in your pond, including shelves. Not only will it help with biological filtration, but the gravel will also help battle “muck” that can build up in ponds over time. Beneficial bacteria love eating organic matter, which includes muck. So although you may still need to clean your pond bottom out once every year or two, putting gravel in the bottom can dramatically cut down on how often you will need to do so. By comparison, a pond without gravel on the bottom may be easier to clean, but much more muck will accumulate because of the lack of natural biological filtration. Another advantage of gravel is it provides a really nice place to grow aquatic plants. In ponds without gravel and rocks, people usually resort to putting pots right into the pond. Firstly, this doesn’t look natural, and secondly, some fish love to dig in potted plants, which can cause a mess. With gravel, you can tuck the roots of the plant right into the gravel or rocks, giving the plant a nice secure location to grow. As a bonus, fish will usually leave a plant alone if it is planted this way.