What’s at the bottom of your pond?

There are many ways to build a koi pond, and some methods can affect your maintenance regimen. Your pond bottom, also known as your substrate, can dramatically change how much time and effort you spend cleaning your pond. We’ll cover some of the most common bottom covers and the pros and cons of each

Gravel/Small Rocks

Pros: This substrate is the most eye appealing of them all. It breaks up the industrial look of a synthetic fish environment and can provide hold for and uncontained plants.

Cons: You will need to stir the gravel/small rocks frequently to keep them from cementing together. A pond vacuum will be of little use and will keep collecting all these small stones. This substrate collects lots of debris and is hard to clean. If the substrate is deeper than 3″, you run the risk of anaerobic bacteria production once they are cut off from the oxygen in the water. This can create pockets of hydrogen sulfide that can kill fish if mixed into the water supply.

Large Rocks

Pros: Another aesthetically appealing bottom substrate. Larger rocks are more suitable to a “river” appearance.

Cons: Your pond vacuum will work slightly better if the substrate is too large to fit up the hose. If the stones are too heavy, you will have a very hard time cleaning between them. Multiple layers of stones will be more likely to collect debris than only one layer of rocks. This layering effect can contribute to cementing of rocks together and formation of hydrogen sulfide producing anaerobic bacteria.

Liner

Pros: A liner bottom is one of the easiest substrates to keep clean. They work well with a bottom drain.

Cons: Pay special attention to the thickness of your liner. Thinner liners may be more economical, but they will not last as long as thicker liners. They are prone to tears from rocks, root growth and boots. They are not as aesthetically pleasing and deep folds can potentially trap fish.

Concrete/Spray Liner

Pros: Concrete bottoms with a spray-applied liner are very easy to keep clean. They are more durable than simple liner ponds. Leaks from tears are not an issue as this type of bottom stands up well to abuse from rocks and boots.

Cons: This is one of the most expensive substrate options for fish ponds. If your concrete is not mixed properly, you will lose chunks of your bottom, leading to leaks. They are susceptible to earthquake damage if this is an environmental concern. Be sure both your concrete mix and spray liner are appropriate for usage with fish. You cannot change the shape of your pond or add any underwater features later with this substrate.

Plants/Mud/Bog/”All Natural”

Pros:

Cons: Your lack of cleaning effort creates a dangerous health hazard to your fish. Dense planting and layers of debris and fish waste create layers of bacteria that lose contact with oxygen in the water. These anaerobic bacteria use sulfur as a food source. If you mess around in the muck or your fish try to flash on this surface, you will release hydrogen sulfide and kill all your fish.

It is impossible to recreate a native environment in a synthetic fish system. “All natural” systems in wild environments use multiple species working together to balance waste management. In a man-made pond, you are missing many of these critical elements. Do your regular maintenance!

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