Want to stir up controversy among pond builders and koi keepers? Ask their opinion on putting gravel on the bottom of the pond.
The anti-gravel camp warns that gravel traps fish waste, while those in favor say it actually prevents muck build-up and improves water quality.
At Splash, we recommend adding a thin layer of gravel to the pond bottom if you have a skimmer. The gravel helps the pond ecosystem break down organic debris that doesn’t make it into the skimmer basket, creating a cleaner pond and keeping your fish happy. Gravel also looks more natural than a bare liner and helps protect that liner from sunlight.
Don’t have a skimmer? First, we strongly recommend installing one so that leaves and other debris don’t sink into the bottom of your pond and start wreaking havoc. But if, for whatever reason, you can’t install a skimmer, then you might be stuck with a bare-liner pond.
Want to know more about how best to use gravel in your pond? Keep reading …
At a Glance: 5 Reasons We Recommend Putting Gravel on the Bottom of Your Pond
- Gravel gives beneficial bacteria a place to colonize. These bacteria break down muck that would otherwise build up on the bottom of the pond.
- Gravel protects plants from curious koi.
- Gravel protects the pond liner from sunlight.
- Gravel looks more natural than a bare liner.
- Gravel makes it easier to work and play in your pond. It’s a lot safer to walk on gravel than on a slippery liner.
*Gravel is best-suited for ponds that have skimmers installed.
Pond Gravel and The Ecosystem Pond
Gravel works best in ponds built with skimmers and biological filtration. We call this kind of water feature an Ecosystem Pond.
An Ecosystem Pond uses manmade tools to replicate the biological processes that happen in natural ponds.
You can read our full guide on Ecosystem Ponds here, but here’s a quick summary of how they work:
- A skimmer removes 80 to 90 percent of the physical debris that enters the pond.
- A biofilter has lots of nooks and crannies where beneficial bacteria colonize. These bacteria help keep your water clear. The water runs up through the biofilter then out through your waterfall.
- Fish-safe EPDM liner keeps water where it’s supposed to be: in your pond.
- Heavy-duty underlayment protects the liner.
- A stepped-shelf design gives you a place to add plants, and your family and pets a way to safely enter and exit the pond.
- Fish caves give your fish a place to get out of the elements and away from predators.
Pond gravel is the final ingredient in a good Ecosystem Pond – and it does several important jobs.
1. Liner protection
A thin layer on the bottom of the pond protects the liner from sunlight.
If you need to get in your pond – for cleaning or even a swim – you’ll have a lot easier time walking on gravel than on a slippery liner.
3. Plant Protection
Gravel protects the pond’s plants from curious koi.
Fish love to play football with potted plants, uprooting the plants and pushing the pots around the pond.
In a gravel-bottom pond, you can plant waterlilies and marginals directly on the liner. Simply remove the plant from its pot and gently press the roots and soil into a plant pocket or against the surface of the liner. Then cover the plant with a thin layer of gravel to keep it in place.
With the plant camouflaged by gravel, fish tend to leave it alone.
Ecosystem Ponds are supposed to look natural, like something you stumbled on while walking in the woods.
Gravel looks a lot more natural than rubber liner.
4. Bacteria Growth
Like filter mats, BioBalls and lava rock in your biofilter, gravel gives beneficial bacteria another place to colonize.
Think about it: In a bare-liner pond, beneficial bacteria only has one two-dimensional surface where it can grow – the top of the liner. In a gravel-bottom pond, by contrast, that bacteria can grow on the top, bottom and sides of every piece of gravel.
More beneficial bacteria in the pond = clearer water and happier fish.
In our more than 30 years’ experience building and designing ponds, we’ve found that gravel-bottom ponds tend to stay significantly cleaner than bare-liner ponds:
- A bare-liner pond without a skimmer might see as much as 12 inches of debris build-up on the bottom of the pond in a year.
- A bare-liner pond with a skimmer might accumulate 2 to 3 inches.
- A gravel-bottom pond with a skimmer has almost zero muck build-up. The skimmer takes out 80 to 90 percent of the debris that enters the pond, and the bacteria on the pond gravel breaks down the other 10 percent.
How Beneficial Bacteria Keep Water Clear
With all this talk about beneficial bacteria, you might wonder, “Isn’t bacteria bad?”
No! At least not this bacteria.
In your pond, beneficial bacteria is key to keeping the water crystal-clear and fish happy and healthy.
Some types break down ammonia, while others help control sludge and green water.
Beneficial bacteria naturally colonizes in all the nooks and crannies in the pond – including the gravel. We always recommend giving the bacteria a boost by adding supplemental doses during the season, either once a week with a pump bottle or via an automatic dosing system.
The more beneficial bacteria you have in your pond, the clearer and healthier your water will be.
Pond gravel facilitates bacteria growth. That’s the No. 1 reason we like it so much.
The Argument Against Pond Gravel
Pond gravel is awesome. So why do so some pond builders argue – often vehemently – against it?
The main argument against gravel-bottom ponds is that all the nooks and crannies act as a magnet for fish waste and other debris.
There is some truth in this – if the pond doesn’t have a skimmer. Without a skimmer, 100 percent of the fish waste, leaves and anything else that ends up in the pond sinks to the bottom.
Beneficial bacteria simply can’t keep up with that much muck. That leaves all that stuff trapped in the gravel, where it decays and wrecks the water quality.
If you don’t have a skimmer, pond gravel probably isn’t a good choice for your pond – but you should really think about installing a skimmer anyway.
Similar to the muck build-up argument, this line of thought goes that thousands of pieces of pond gravel are a lot harder to clean than one smooth liner.
It makes sense on the surface – except, as we’ve mentioned, gravel-bottom ponds actually stay a lot cleaner than their bare-liner counterparts.
Most ponds do need a clean-out every one to three years, regardless of what’s on the bottom of the pond.
When clean-out time comes, you won’t be scrubbing every individual rock. The goal instead is to do a quick once-over with a hose to dislodge big debris (we’ll get into more details on cleaning your gravel pond below).
Koi are natural foragers. They use those little barbels protruding from their faces to root around rocks and find algae and small critters to eat.
Anti-gravel folks argue this behavior leads fish to indiscriminately eat anything small enough to fit in their mouths – including gravel.
While it’s true that koi will eat just about anything, including smaller fish, we’ve found gravel-related injuries are pretty rare.
To keep fish – and the liner – safe, only use smooth-edged rocks and gravel in your pond.
Some people argue that gravel is an unnecessary expense that pond builders only push so they can jack up your bill.
While you could save money by skipping the skimmer and gravel, you’ll find that this combination saves you a lot of maintenance headaches – and expenses – in the long run.
Pond gravel typically makes up a small portion of the overall cost of building a pond. Ours costs $99 per skid loader scoop (about 1 ton).
Considering ponds only needs a thin layer of gravel (no more than 2-3 inches), and the smallest DIY pond kits start around $1,100, the cost of gravel is barely a drop in the bucket of the overall cost.
How to Clean Pond Gravel
One of the most common questions pond owners ask when considering whether they want gravel is, “How do I clean it?”
Cleaning pond gravel is something you can easily do as part of an annual clean-out.
During a clean-out, you (or the crew you hire) will move fish to a temporary holding tank and drain the pond using a pump and discharge hose. Once you get to the bottom of the pond, simply use the old pond water – and a little fresh hose water as-needed – to gently disturb the gravel and dislodge large debris.
The goal here is to get rid of muck and leaves, not sanitize the surface of every rock. That’s because these surfaces grow the kinds of bacteria that keep your water clear. The more of that biofilm you can keep in the pond, the better.
Most pond owners do this kind of full clean-out every one to three years. For the times in between cleanings, you can use a contact algaecide to kill string algae on gravel, and a good skimming net to pull out large debris.